ON THE

O R I G I N O F SPECIES.

“ But with regard to the material world, we can at least go so
far as this-we can perceive that events are brought about not by
insulated interpositions of Divine power, exerted in each particularcase, but by the establishment of general laws.”

W. WREWELL: Bridgewater Treatise.

“To conclude, therefore, let no man out of a weak conceit ofsobriety, or an ill-applied moderation, think or maintain, that a
man can search too far or be too well studied in the book of God‘sword, or in the book of God’s works ; divinity or philosophy ; butrather let inen endeavour an endless progress or proficience in both.”

BACON; Advancement of Learning.

Down, Bromley, Kmt,
October lst, 1859.

ON

THE O R I G I N OF SPECIESBY MEANS OF NATURAL SELECTION,

OR THEPRESERVATION OF FAVOURED RACES IN THE STRUGGLE

FOR LIFE.

BY CHARLES DARWIN, M A ,FELLOW OF T H E R O Y A L , GEOIJOGICA11, LlNNA%AN, ETC., SOCIETIES;AUTHOR OF 'JOURNAL OF RESEARCHES DURING XI. M. s. BEAOLE'EIVOYAGEROUND THE WORLD.'LONDON:JOHN MURRAY, ALBEMARLE STREET.
1859.

The right of WamMion is reaei-tw?d.

LONDON: PRINTED XP W. CWWES AND SONU, STAMFORD STREET.
AND CFIARING CROSS.

C 0 N T E N T S.

INTRODUCTION. . . . . . . . . . . . . . . . . . Page 1

C H A P T E R I.
VARIATIONUNDER DOMESTICATION.
Causes of Variability -Effects of Habit -Correlation of Growth Inheritance -Character of Domestic Varieties -Difficultyof
distinguishing between Varieties and Species-Origin of Domestic
Varieties from one or more Species -Domestic Pigeons, theirDifferences and Origin -Principle of Selection anciently followed,
its Effects -Methodical 2nd Unconscious Selection -Unknown
Origin of our Domestic Productions -Circumstances favourable
to Man’s powerof Selection . . . . . . . . . . . .
7-43

C H A P T E R 11.
VARIATION
UNDER NATURE.
Variability - Individual differences - Doubtful species - Wideranging, much diffused, and common species vary m o s t S p e cies of the larger genera in any country vary niore tban the speciesof the smaller genera-Many of the species of the larger genera
resemble varieties in being very closely, but unequally, related
to each other, and in having restricted ranges. . . . 44-59

vi

CONTENTS.

C H A Y T E B 111.
STRUGGLE
FOR EXISTEHCY.
Bears on natural selection-The term used in a wide sense-Geometrical powers of increase - napid iiicrcxse of naturalised
animals and plants-Nature of the checks to incrcase-Competition universal - Effects of climate - Protection from thenumber of individuals-complex relations of all animals andplants throughout nature-Struggle for life most severe between
individuals and varieties of the same species ; often severe between species of the same genus-The relation of organism toorganism the most important of all relations,.
Page 60-79

CHAPTER IV.
NATURA
I, SELECTION.
Natural Selection - its power compared with man's selection- its
power on characters of trifling importance -its power at all ages
and on both sexes - Sexual Selection -On the gcnerality of intercrosses between individuals of the same species -Circnmstancea
favourable and unfavourable to Natwal Selection, namely,intercrossing, isolation, number of individuals - Slow action Extinction caused by Natural Selection -Divergence of Character, related to the diversity of inhabitants of any small area,
and to naturalisation -Action of Natural Selection, throughDivergence of Character and Extinction, on the descendants
from a common parent -Explains the Grouping of all organic
beings .. .. .. . . . . . . .. .. . . . . 80-130

C H A P T E R V.
LAWSOF VARIATION.
Effects of external conditions -Use and disuse, combined withnatural selection ; organs of flight and of vision - Acclimatisation -Correlation of growth -Compensation and economy ofgrowth -False correlations -Multiple, rudimenbary, and lowly
organised structures variable -Parts developed in an unusual
manner are highly variable : specific characters more variable
than generic : secondary sexual characters variable -Species ofthe same genus vary in an analogous manner -Reversions to.. .. .. 131-170
long-lost characters-Summary .

. ..

CONTENTS.

vii

C H A P T E R VI.

DIFFICULTIESON THEORY.
Difficultieson the theory of descent with modification-TransitionsAbsence or rarity of transitional varieties-Transitions in habitsof life-Diversified habits in the same species-Species with
habits widely different from those of their allies-Organs ofcxtrcme perfection-Means of transition-Cases of difficultyNatura non facit saltum-Organs of small importance-Organs
not in all cases absolutely perfect-The
law of Unity of Typeand of the Conditions of Existence embraced by the theory of
Natural Selection.. .. .. . . .. .. Page 171-206

CHAPTER VII.
INSTINCT.
Instincts comparable with habits, but different in their origin Instincts graduated - Aphides and ants -Instincts variable Domestic instincts, their origin -Natural instincts of the cuckoo,
ostrich, and parasitic bees - Slave-making ants -Hive-bee, its
cell-making instinct - Difficulties on the theory of the Natural
Selection of instincts - Neuter or sterile insects - Scmmary
207-244

CHAPTER VIII.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids Sterility various in degree, not universal, affected by close interbreeding, removed by domestication-Laws governing the sterilityof hybrids - Sterility not a special endowment, but incidental
on other differences - Causes of the sterility of first crosses andof hybrids - Parallelism between the effects of changed conditions of life and crossing - Fertility of varieties when crossed
and of thcir mongrcl offspring not universal - Hybrids andmongrels compared indelmndently of their fertility - Summary
245-278

...

Vlll

CONTENTS.

C H A P T E H IX.
ON THE ~MPERFECTION OF THE GEOLOGICALRECORD.
On the absence of intermediate varieties at the present day -On
the nature of extinct intermediate varieties ; on their number On the vast lapse of time, as inferred from the rate of deposition and of denudation - On the poorness of our palsontologicalcollections - On the intermittence of geological formations On the absence of intermediate varieties in any one formation-On the sudden appearance of groups of species - On theirsudden appearance in the lowest known fossiliferous strata
Page 279-311

C H A P T E R X.
ON THE GEOLOGICALSUCCESSION
OF ORGANICBEINGS.
On the slow and successive appearance of new species - On theirdifferent rates of change -Species once lost do not reappear Groups of species follow the same general rules in their appearance and disappearance as do single species - On Extinction On simultaneous changes in the forms of life throughout theworld - On the affinities of extinct species to each other and toliving species -3 n the state of development of ancient forms On the succession of the same types within the same areas Summary of preceding and present chapters., .. 312-345

C H A P T E R XI.
GEOGRAPHICALDISTRIBUTION.

Present distribution cannot be accounted for by differences in physical couditions - Importance of barriers -Affinity of the productions of the same continent -Centres of creation -Meansof dispersal, by changes of climate and of the level of the land,and by occasional means -Dispersal during the Glacial period346-382
co-extensive with the world .. .. .. .

. .. ..

CONTENTS.

ix

CHAPTER XII.
GEOGRAPRICAL
DISTRIBUTION-continzced.
Distribution of fresh-water productions -On the inhabitants ofoceanic islands -Absence of Batrachians and of terrestrial Mammals -On the relation of the inhabitants of islands to those of
the nearest mainland -On colonisation from the nearest sourcewith subsequent modification - Summary of the last and pre. ..
.
Page 3ti3-410
sent chapters.. .

.. . .. ..

CHAPTER XIII.
MUTUALAFFINITIESOF ORGANICBEINGS: MORPHOLOGY:EMBRYOLOGY: RUDIXENTARYORGANS.
CLASSIFICATION,
groups subordinate to groups -Natural system Rules and difficulties in classification, explained on tbe theory ofdescent with modification -Classification of varieties -Descentalways used in classification -Analogical or adaptive characters-Affinities, general, complex and radiating -Extinction separates and defines groups -MORPHOLOGY,between membersof the same class, between parts of the same individualEMBRYOLOGY,
laws of, explained by variations not superveningat an early age, and being inherited at a corresponding age
RUDIMENTARYORGANS ; their origin explained -Summary
411-458-

CHAPTER XIV.
RECAPITULATIONAND CONCLUSION.
Recapitulation of the difficulties on the theory of Natural Selection-Recapitulation of the general and special circumstances in its
favour -Causes of the general belief in the immutability of
species-How
far the theory of natural selection may be
extended-Effects
of its adoption on the study of Natural,
459-490
history Concluding remarks.. .. .. . ..
INDEX .. .. .. .. .. .. .. .. .. .. .. .. 491

ON THE ORIGIN OF SPECIES.

I N T R O D U C TION.

WHENon board H.M.S. ‘Beagle,’ as naturalist, I wasmuch struck with certain facts in the distribution of the
inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some lighton the origin of species-that mystery of mysteries, as
it has been called by one of our greatest philosophers.
On my return home, it occurred to me, in 1837, thatsomething might perhaps be made out on this question
by patiently accumulating and reflecting on all sorts offacts which could possibly have any bearing on it. After
five years’ work I allowed myself to speculate on thesubject, and drew up some short notes ; these I enlarged
in 1844 into a sketch of the conclusions, which thendeemed to me probable : from that period to the present
day I have steadily pursued the same object. I hopethat I may be excused for entering on these personal
details, as I give them to show that I have not been hastyin coming to a decision.
My work is now nearly finished ; but as it will takeme two or three more years to complete it, and as myhealth is far from strong, I have been urged to publish
this Abstract. I have more especially been induced
to do this, as Mr. Wallace, who is now studying the2

INTRODUCTION.

natural history of the Malay archipelago, has arrived
at almost exactly the same general conclusions that I
have on the origin of species. Last year he sent to mea memoir on this subject, with a request that I would
forward it to Sir Charles Lyell, who sent it to the Linnean Society, and it is published in the third volume of
the Journal of that Society. Sir C. Lyell and Dr.
Hooker, who both knew of my work-the latter having
read my sketch of 1844-honoured me by thinking it
advisable to publish, with Mr. Wallace’s excellent memoir, some brief extracts from my manuscripts.
This Abstract, which I now publish, must necessarily
be imperfect. I cannot here give references and authorities for my several statements; and I must trust to
the reader reposing some confidence in my accuracy.
No doubt errors will have crept in, though I hope I havealways been cautious in trusting to good authorities
alone. . I can here give only the general conclusions at
which I have arrived, with a few facts in illustration,
but which, I hope, in most cases will suffice. No one
can feel more sensible than I do of the necessity of hereafter publishing in detail all the facts, with references,on which my conclusions have been grounded; and I
hope in a future work to do this. For I am well awarethat scarcely a single point is discussed in this volume
on which facts cannot be adduced, often apparently
leading to conclusions directly opposite to those at which
I have arrived. A fair result can be obtained only by
fully stating and balancing the facts and arguments on
both sides of each question ; and this cannot possibly behere done.
I much regret that want of space prevents my having
the satisfaction of acknowledging the generous assistancewhich I have received from very many naturalists, someof them personally unknown to me. I cannot, however,INTRODUCTION.

3

let this opportunity pass without expressing my deep
obligations to Dr. Hooker, who for the last fifteen years
has aided me in every possible way by his large storesof knowledge and his excellent judgment.
I n considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual
affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, night come to the conclusionthat each species had not been independently created,but had descended, like varieties, from other species.
Nevertheless, such a conclusion, even if well founded,
would be unsatisfactory, until it could be shown how
the innumerable species inhabiting this world have been
modified, so as to acquire that perfection of structure
and coadaptation which most justly excites our admiration. Naturalists continually refer to external conditions, such as climate, food, &c., as the only possible
cause of variation. I n one very limited sense, as weshall hereafter see, this may be true; but it is preposterous to attribute to mere external conditions, thestructure, for instance, of the woodpecker, with its feet,tail, beak, and tongue, so admirably adapted to catch
insects under the bark of trees. I n the case of themisseltoe, which draws its nourishment from certain
trees, which has seeds that must be transported bycertain birds, and which has flowers with separate sexes
absolutely requiring the agency of certain insects tobring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite,with its relations to several distinct organic beings, by
the effects of external conditions, or of habit, or of thevolition of the plant itself.
The author of the ‘Vestiges of Creation’ would, I
presume, say that, after a certain unknown number of

4

INTRODUCTION.

generations, some bird had given birth to a woodpecker,
and some plant to the misseltoe, and that these had
been produced perfect as we now see them; but thisassumption seems to me to be no explanation, for it
leaves the case of the coadaptations of organic beings toeach other and to their physical conditions of life, untouched and unexplained.
It is, therefore, of the highest importance to gain aclear insight into the means of modification and coadaptation. At the commencement of my observations
it seemed to me probable that a careful study of domesticated animals and of cultivated plants would offer thebest chance of making out this obscure problem. Nor
have I been disappointed; in this and in all otherperplexing cases I have invariably found that ourknowledge, imperfect though it be, of variation underdomestication, afforded the best and safest clue. I may
venture to express my conviction of the high d u e ofsuch studies, although they have been very commonly
neglected by naturalists.
From these considerations, I shall devote the firstchapter of this Abstract tovariation under Domestication.
We shall thus see that a large amount of hereditarymodification is at least possible ; and, what is equally or
more important, we shall see how great is the power ofman in accumulating by his Selection succesive slightvariations. I will then pass on to the variability of
species in a state of nature ; but I shall, unfortunately,
be compelled to treat this subject far too briefly, m it
can be treated properly only by giving long catalogues
of facts. We shall, however, be enabled to discuss
what circumstances are most favourable to variation.
In the next chapter the Struggle for Existence amongst
all organic beings throughout the world, which inevitably follows from their high geometrical powers of

INTRODUCTION.

5

increase, will be treated of. This is the doctrine ofMalthus, applied to the whole animal and vegetable
kingdoms. As many more individuals of each species
are born than can possibly survive; and as, consequently, there is a frequently recurring struggle forexistence, it follows that any being, if it vary howeverslightly in any manner profitable to itself, under thecomplex and sometimes varying conditions of life, will
have a better chance of surviving, and thus be natwally
selected. From the strong principle of inheritance, any
selected variety will tend to propagate its new andmodified form.
This fundamental subject of Natural Selection willbe treated at some length in the fourth chapter ; and weshall then see how Natural Selection almost inevitablycauses much Extinction of the less improved forms oflife, and induces what I have called Divergence ofCharacter. I n the next chapter I shall discuss thecomplex and little known laws of variation and of correlation of growth. I n the four succeeding chapters, themost apparent and gravest difficulties on the theory
will be given: na.mely, first, the difficulties of transitions, or in understanding how a simple being or a simple
organ can be changed and perfected into a highly
developed being or elaborately constructed organ ;secondly, the subject of Instinct, or the mental powers ofanimals ; thirdly, Hybridism, or the infertility of species
and the fertility of varieties when intercrossed; andfourthly, the imperfection of the Geological Record.
In the next chapter I shall consider the geologicalsuccession of organic beings throughout time; in theeleventh and twelfth, their geographical distribution
throughout space ; in the thirteenth, their classification
or mutual affinities, both when mature and in an embryonic condition. I n the last chapter I shall give a

6

INTRODUCTION.

brief recapitulation of the whole work, and a few concluding remarks.
No one ought to feel surprise at much remaining as
yet unexplained in regard to the origin of species and~arieties,if he makes due allowance for our profoundignorance in regard to the mutual relations of all the
beings which live around us. Who can explain why one
species ranges widely and is very numerous, and why
another allied species has a narrow range and is rare?
Yet these relations are of the highest importance, forthey determine the present welfare, and, as I believe,
the future success and modification of every inhabitantof this world. Still less do we know of the mutual
relations of the innumerable inhabitants of the world
during the many past geological epochs in its history.
Although much remains obscure, and will long remain
obscure, I can entertain no doubt, after the most deliberate study and dispassionate judgment of which I
am capable, that the view which most naturalists entertain, and which I formerly entertained-namely, thateach species has been independently created-is erroneous. I am fully convinced that species are not immutable ; but that those belonging to what are called
the same genera are lineal descendants of some otherand generally extinct species, in the same manner as
the acknowledged varieties of any one species are thedescendants of that species. Furthermore, I am convinced that Natural Selection has been the main but
not exclusive means of modification.

CHAP. I.

VARIATION UNDER DOMESTICATION.

7

C H A P T E R I.
VARIATIONUNDER DOMESTICATION.
Causes of Variability-Effects of Habit-Correlation of GrowthInheritance Character of Domestic Varieties Difficulty of
distinguishing between Varieties and Species-Origin of Domestic
Varieties from one or more Species-Domestic Pigeons, theirDifferences and Origin-Principle of Selection anciently followed,
its Effects-Methodical and Unconscious Selection-Unknown
Origin of our Domestic Productions-Circumstances favourable
to Man’s power of Selection.

--

WHENwe look to the individuals of the same variety or
sub-variety of our older cultivated plants and animals,
one of the first points which strikes us, is, that theygenerally differ much more from each other, than do theindividuals of any one species or variety in a state ofnature. When we reflect on the vast diversity of theplants and animals which have been cultivated, andwhich have varied during all ages under the mostdifferent climates and treatment, I think we are driven
to conclude that this greater variability is simply due toour domestic productions having been raised under conditions of life not so uniform as, and somewhat different
from, those to which the parent-species have been exposed
under nature. There is, also, I think, some probability
in the view propounded by Andrew Knight, that thisvariability may be partly connected with excess of food.
It seems pretty clear that organic beings must be exposed during several generations to the new conditionsof life to cause any appreciable amount of variation;and that when the organisation has once begun to vary,it generally continues to vary for many generations.

8

VARIATION

CHAP. I.

No case is on record of a variable being ceasing to bevariable under cultivation. Our oldest cultivated plants,
such as wheat, still often yield new varieties : our oldest
domesticated animals are still capable of rapid improvement or modification.
It has been disputed at what period of life the causesof variability, whatever they may be, generally act;
whether during the early or late period of developmentof the embryo, or at the instant of conception. Geoffroy
St. Hilaire’s experiments show that unnatural treatmentof the embryo causes monstrosities ; and monstrosities
cannot be separated by any clear line of distinction
from mere variations. But I am strongly inclined tosuspect that the most frequent cause of variability may
be attributed to the male and female reproductive
elements having been affected prior to the act of conception. Several reasons make me believe in this ; but
the chief one is the remarkable effect which confinement or cultivation has on the functions of the reproductive system; this system appearing to be far more
susceptible than any other part of the organisation, tothe action of any change in the conditions of life. Nothing is more easy than to tame an animal, and few
things more difficult than to get it to breed freely underconfinement, even in the many cases when the male andfemale unite. How many animals there are which will
not breed, though living long under not very close confinement in their native country! This is generally
attributed to vitiated instincts ; but how many cultivatedplants display the utmost vigour, and yet rarely or neverseed! I n some few such cases it has been found outthat very trifling changes, such as a little more or lesswater at some particular period of growth, will determine
whether or not the plant sets a seed. I cannot here
enter on the copious details which I have collected on

CHAP. I.

UNDER DOMESTICATION.

9

this curious subject; but to show how singular the laws
are which determine the reproduction of animals underconfinement, I may just mention that carnivorous animals,
even from the tropics, breed in this country pretty
freely under confinement, with the exception of theplantigrades or bear family ; whereas, carnivorous birds,
with the rarest exceptions, hardly ever lay fertile eggs.
Many exotic plants have pollen utterly worthless, in thesame exact condition as in the most sterile hybrids.
When, on the one hand, we see domesticated animals
and plants, though often weak and sickly, yet breedingquite freely under confinement ; and when, on the other
hand, we see individuals, though taken young from astate of nature, perfectly tamed, long-lived, and healthy(of which I could give numerous instances), yet having
their reproductive system so seriously affected by unperceived causes as to fail in acting, we need not be
surprised at this system, when it does act under confinement, acting not quite regularly, and producing offspring not perfectly like their parents or variable.
Sterility has been said to be the bane of horticulture ;but on this view we owe variability to the same cause
which produces sterility ; and variability is the source of
all the choicest productions of the garden. I may add,
that as some organisms will breed most freely underthe most unnatural conditions (for instance, the rabbit
and ferret kept in hutches), showing that their reproductive system has not been thus affected ; so will someanimals and plants withstand domestication or cultivation, and vary very slightly-perhaps hardly more thanin a state of nature.
A long list could easily be given of “sporting plants;”
by this term gardeners mean a single bud or offset,
which suddenly assumes a new and sometimes very different character from that of the rest of the plant.

10

VARIATION

CHAP.I.

Such buds can be propagated by grafting, &c., andsometimes by seed. These sports ” are extremelyrare under nature, but far from rare under cultivation ;and in this case we see that the treatment of the parent
has affected a bud or offset, and not the ovules or pollen,
But it is the opinion of most physiologists that there is no
essential difference between a bud and an ovule in theirearliest stages of formation ; so that, in fact, ‘(sports”
support my view, that variability may be largely attributed to the ovules or pollen, or to both, having been
affected by the treatment of the parent prior to the act ofconception. These cases anyhow show that variation is
not necessarily connected, as some authors have supposed, with the act of generation.
Seedlings from the same fnrit, and the young of thesame litter, sometimes differ considerably from eachother, though both the young and the parents, as Miiller
has remarked, have apparently been exposed to exactly
the same conditions of life ; and this shows how unimportant the direct effects of the conditions of life are incomparison with the laws of reproduction, and of growth,and of inheritance ; for had the action of the conditions
been direct, if any of the young had varied, all would
probably have varied in the same manner. To judge how
much, in the case of any variation, we should attributeto the direct action of heat, moisture, light, food, &c.,
is most difficult : my impression is, that with animals
such agencies have produced very little direct effect,
though apparently more in the case of plints. Under
this point of view, Mr. Buckman’s recent experimentson plants seem extremely valuable. When all or nearly
all the individuals exposed to certain conditions me
affected in the same way, the change at first appears to
be directly due to such conditions ; but in some cases it
can be shown that quite opposite conditions produce((

CHAP. I.

UNDER DOMESTICATION.

11

similar changes of structure. Nevertheless some slightamount of change may, I think, be attributed to thedirect action of the conditions of life-as, in some cases,
increased size from amount of food, colour from particular kinds of food and from light, and perhaps the
thickness of fur from climate.
Habit also has a decided influence, as in the periodof flowering with plants when transported from one
climate to another. I n animals it has a more marked
effect; for instance, I find in the domestic duck that thebones of the wing weigh less and the bones of the leg
more, in proportion to the whole skeleton, than do thesame bones in the wild-duck ; and I presume that thischange may be safely attributed to the domestic duckflying much less, and walking more, than its wild parent.
The great and inherited development of the udders incows and goats in countries where they are habitually
milked, in comparison with the state of these organs
in other countries, is another instance of the effect ofuse. Not a single domestic animal can be named
which has not in some country drooping ears; and theview suggested by some authors, that the drooping is dueto the disuse of the muscles of the ear, from the animals
not being much alarmed by danger, seems probable.
There are many laws regulating variation, some fewof which can be dimly seen, and will be hereafter briefly
mentioned. I will here only allude to what may becalled correlation of growth. Any change in the embryoor larva will almost certainly entail changes in themature animal. In monstrosities, the correlations between quite distinct parts are very curious ; and many
instances are given in Isidore Geoffroy St. Hilaire’s great
work on this subject. Breeders believe that long limbs
are almost always accompanied by an elongated head.
Some instances of correlation are quite whimsical : thus

12

VARIATION

CHAP. I.

cats with blue eyes are invariably deaf; colour and constitutional peculiarities go together, of which many
remarkable cases could be given amongst animals andplants. From the facts collected by Heusinger, it appears that white sheep and pigs are differently affectedfrom coloured individuals by certain vegetable poisons.
Hairless dogs have imperfect teeth ; long-haired andcoarse-haired animals are apt to have, as is asserted, longor many horns; pigeons with feathered feet have skinbetween their outer toes ; pigeons with short beaks havesmall feet, and those with long beaks large feet. Hence,
if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly unconsciously modify
other parts of the structure, owing to the mysterious
laws of the correlation of growth.
The result of the various, quite unknown, or dimly
seen laws of variation is infinitely complex and diversified.
It is well worth while carefully to study the several
treatises published on some of our old cultivated plants,
as on the hyacinth, potato, even the dahlia, &c. ; and it
is really surprising to note the endless points in structure and constitution in which the varieties and subvarieties diff'er slightly from each other. The wholeorganisation seems to have become plastic, and tends todepart in some small degree from that of the parental
type.
Any variation which is not inherited is unimportantfor us. But the number and diversity of inheritabledeviations of structure, both those of slight and those of
considerable physiological importance, is endless. Dr.
Prosper Lucas's treatise, in two large volumes, is thefullest and the best on this subject. No breeder doubts
how strong is the tendency to inheritance : like produces
like is his fundamental belief: doubts have been thrownon this principle by theoretical writers alone. When aCHAP. I.

UNDER DOMESTICATION.

13

deviation appears not unfrequently, and we see it in thefather and child, we cannot tell whether it may not be
due to the same original cause acting on both ; but whenamongst individuals, apparently exposed to the same
conditions, any very rare deviation, due to some extraordinary combination of circumstances, appears in theparent-say, once amongst several million individualsand it reappears in the child, the mere doctrine ofchances almost compels us to attribute its reappearance
to inheritance. Every one must have heard of cases ofalbinism, prickly skin, hairy bodies, &c., appearing inseveral members of the same family. If strange andrare deviations of structure are truly inherited, lessstrange and commoner deviations may be freely admitted to be inheritable. Perhaps the correct way ofviewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, andnon-inheritance as the anomaly.
The laws governing inheritance are quite unknown ;no one can say why the same peculiarity in different
individuals of the same species, and in individuals ofdifferent species, is sometimes inherited and sometimes
not so ; why the child often reverts in certain charactersto its grandfather or grandmother or other much more
remote ancestor ; why a peculiarity is often transmitted
from one sex to both sexes, or to one sex alone, morecommonly but not exclusively to the like sex. It isa fact of some little importance to us, that peculiarities appearing in the males of our domestic breeds
are often transmitted either exclusively, or in a muchgreater degree, to males alone. A much more important rule, which I think may be trusted, is that, at
whatever period of life a peculiarity first appears, it
tends to appear in the offspring at a corresponding age,though sometimes earlier. In many cases this could

14

VARIATION

CHAP. I.

not be otherwise : thus the inherited peculiarities in thehorns of cattle could appear only in the offspring whennearly mature ; peculiarities in the silkworm are known
to appear at the corresponding caterpillar or cocoon
stage. But hereditary diseases and some other facts
make me believe that the rule has a wider extension,and that when there is no apparent reason why a peculiarity should appear at any particular age, yet that it
does tend to appear in the offspring at the same period
at which it first appeared in the parent. I believe thisrule to be of the highest importance in explaining thelaws of embryology. These remarks are of course confined to the fist appearance of the peculiarity, and not
to its primary cause, which may have acted on theovules or male element ; in nearly the same manner asin the crossed offspring from a short-horned cow by along-horned bull, the greater length of horn, thoughappearing late in life, is clearly due to the maleelement.
Having alluded to the subject of reversion, I may
here refer to a statement often made by naturalistsnamely, that our domestic varieties, when run wild,
gradually but certainly revert in character to theiraboriginal stocks. Hence it has been argued that no
deductions can be drawn from domestic races to species
in a state of nature. I have in vain endeavoured to
discover on what decisive facts the above statement has
so often and so boldly been made. There would begreat difficulty in proving its truth : we may safely conclude that very many of the most strongly-marked
domestic varieties could not possibly live in a wild state.
In many cases we do not know what the aboriginal stock
was, and so could not tell whether or not nearly perfect
reversion had ensued. It would be quite necessary, inorder to prevent the effects of intercrossing, that only aCHAP. I.

UNDER DOMESTICATION.

15

single variety should be turned loose in its new home.
Nevertheless, as our varieties certainly do occasionally
revert in some of their characters to ancest>ralforms, it
seems to me not improbable, that if we couldsucceed ~ L T
naturalising, or were to cultivate, during many generations, the several races, for instance, of the cabbage, invery poor soil (in which case, however, some effect would
have to be attributed to the direct action of the poorsoil), that they would to a large extent, or even wholly,
revert to the wild aboriginal stock. Whether or not theexperiment would succeed, is not of great importancefor our line of argument; for by the experiment itself
the conditions of life are changed. If it could be shownthat our domestic varieties manifested a strong tendency
to reversion,-that is, to lose their acquired characters,
whilst kept under unchanged conditions, and whilst kept
in a considerable body, so that free intercrossing might
check, by blending together, any slight deviations ofstructure, in such case, I grant that we could deduce
nothing from domestic varieties in regard to species.
But there is not a shadow of evidence in favour of thisview: to assert that we could not breed our cart andrace-horses, long and short-horned cattle, and poultryof various breeds, and esculent vegetables, for an almost
infinite number of generations, would be opposed to all
experience. I may add, that when under nature theconditions of life do change, variations and reversionsof character probably do occur ; but natural selection,
as will hereafter be explained, will determine how far
the new characters thus arising shall be preserved.
When we look to the hereditary varieties or races ofour domestic animals and plants, and compare them withspecies closely allied together, we generally perceive ineach domestic race, as already remarked, less uniformityof character than in true species. Domestic races of

16

VARIATION

CEAP. I.

the same species, also, often have a somewhat monstrous
character ; by which I mean, that, although differingfrom each other, and from the other species of the same
genus, in several trifling respects, they often differ in an
extreme degree in some one part, both when compared
one with another, and more especially when comparedwith all the species in nature to which they are nearest
allied. With these exceptions (and with that of theperfect fertility of varieties when crossed,ya subject
hereafter to be discussed), domestic races of the same
species differ &om each other in the same manner as,
only in most cases in a lesser degree than, do closelyallied species of the same genus in a state of nature. I
think this must be admitted, when we find that there are
hardly any domestic races, either amongst animals orplants, which have not been ranked by some competentjudges as mere varieties, and by other competent judges
as the descendants of aboriginally distinct species. If
any marked distinction existed between domestic races
and species, this source of doubt could not so perpetually recur. It has often been stated that domestic races
do not differ from each other in characters of genericvalue. I think it could be shown that this statement ishardly correct ; but naturalists differ most widely indetermining what characters are of generic value ; allsuch valuations being at present empirical. Moreover,on the view of the origin of genera which I shall presently give, we have no right to expect often to meetwith generic differences in our domesticated productions.
When we attempt to estimate the amount of structuraldifference between the domestic races of the same species,
we are soon involved in doubt, from not knowing whether
they have descended from one or several parent-species.
This point, if it could be cleared up, would be interesting; if,for instance, it could be shown that, the grey-CHAP. I.

UNDER DOMESTICATION.

17

hound, bloodhound, terrier, spaniel, and bull-dog, which
we all know propagate their kind so truly, were the offspring of any single species, then such facts would havegreat weight in making us doubt about the immutabilityof the many very closely allied and natural species-for
instance, of the many foxes-inhabiting different quartersof the world. I do not believe, as we shall presently
see, that all our dogs have descended from any one
wild species; but, in the case of some other domestic
races, there is presumptive, or even strong, evidence infavour of this view.
It has often been assumed that man has chosen fordomestication animals and plants having an extraordinary inherent tendency to vary, and likewise towithstand diverse climates. I do not dispute that thesecapacities have added largely to the value of most of
our domesticated productions ; but how could a savage
possibly know, when he h s t tamed an animal, whether
it would vary in succeeding generations, and whether it
would endure other climates ? Has the little variabilityof the ass or guinea-fowl, or the small power of enduranceof warmth by the rein-deer, or of cold by the common
camel, prevented their domestication? 1 cannot doubtthat if other animals and plants, equal in number toour domesticated productions, and belonging to equally
diverse classes and countries, were taken from a stateof nature, and could be made to breed for an equal
number of generations under domestication, they would
vary on an average as largely as the parent species ofour existing domesticated productions have varied.
I n the case of most of our anciently domesticatedanimals and plants, I do not think it is possible to cometo any definite conclusion, whether they have descended
from one or several species. The argument mainly
relied on by those who believe in the multiple origin

18

VARIATION

CHAP. I.

of our domestic animals is, that we find in the mostancient records, more especially on the monuments of
Egypt, much diversity in the breeds ; and that some of
the breeds closely resemble, perhaps are identical with,those still existing. Even if this latter fact were
found more strictly and generally true than seems
to me to be the case, what does it show, but that someof our breeds originated there, four or five thousand
years ago? But Mr. Horner’s researches have rendered it in some degree probable that man sufficiently
civilized to have manufactured pottery existed in thevalley of the Nile thirteen or fourteen thousand years ago ;and who will pretend to say how long before these ancientperiods, savages, like those of Tierra del Fuego or Australia, who possess a semi-domestic dog, may not haveexisted in Egypt ?
The whole subject must, I think, remain vague;
neverthelsss, I may, without here entering on any
details, state that, from geographical and other considerations, I think it highly probable that our domestic
dogs have descended from several wild species. I n
regard to sheep and goats I can form no opinion. I
should think, from facts communicated to me by Mr.
Blyth, an the habits, voice, and constitution, &c., of the
humped Indian cattle, that these had descended froma different aboriginal stock from our European cattle ;and several competent judges believe that these latter
have had more than one wild parent. With respect tohorses, from reasons which I cannot give here, I am
doubtfully inclined to believe, in opposition to several
authors, that all the races have descended from one
wild stock. Mr. Blyth, whose opinion, from his large
and varied stores of knowledge, I should value morethan that of almost any one, thinks that all the breedsof poultry have proceeded from the common wild

CHAP. I.

UNDER DOMESTICATION.

19

Indian fowl (Gallus bankiva). I n regard to ducks andrabbits, the breeds of which differ considerably fromeach other in structure, I do not doubt that they all
have descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic
races from several aboriginal stocks, has been carried toan absurd extreme by some authors. They believe thatevery race which breeds true, let the distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a scor’: ofspecies of wild cattle, as many sheep, and several goatsin Europe alone, and several even within Great Britain.
One author believes that there formerly existed in
Great Britain eleven wild species of sheep peculiar to it !
When we bear in mind that Britain has now hardly one
peculiar mammal, and France but few distinct from thoseof Germany and conversely, and so with Hungary,
Spain, &c., but that each of these kingdoms possesses
several peculiar breeds of cattle, sheep, &c., we must
admit that many domestic breeds have originated inEurope ; for whence could they have been derived, as
these several countries do not possess a number of
peculiar species as distinct parent-stocks? So it is in
India. Even in the case of the domestic dogs of thewhole world, which I fully admit have probably descended from several wild species, I cannot doubt that
there has been an immense amount of inherited variation. Who can believe that animals closely resembling
the Italian greyhound, the bloodhound, the bull-dog,or Blenheim spaniel, &c.-so unlike all wild Canidae-ever existed freely in a state of nature? It hasoften been loosely said that all our races of dogs havebeen produced by the crossing of a few aboriginal
species ; but by crossing we can get only forms in somedegree intermediate between their parents; and if we20

DOMESTIC PIGEONS.

CHAP. I.

account for our several domestic races by this process,we must admit the former existence of the most extreme
forms, as the Italian greyhound, bloodhound, bull-dog,&c., in the wild state. Moreover, the possibility ofmaking distinct races by crossing has been greatly exaggerated. There can be no doubt that a race may be
modified by occasional crosses, if aided by the carefulselection of those individual mongrels, which present any
desired character; but that a race could be obtained
nearly intermediate between two extremely different
races or speceies, I can hardly believe. Sir J. Sebright
expressly experimentised for this object, and failed. The
offspring from the fist cross between two pure breeds
is tolerably and sometimes (as I have found withpigeons) extremely uniform, and everything seems simple
enough ; but when these mongrels are crossed one withanother for several generations, hardly two of them willbe alike, and then the extreme difficulty, or rather utter
hopelessness, of the task becomes apparent. Certainly,a breed intermediate between two very distinct breeds
could not be got without extreme care and long-conh u e d selection ; nor can I find a single case on recordof a permanent race having been thus formed.
On the Breed8 of the Domestic Pigeon.-Believingthat it is always best to study some special group, I
have, after deliberation, taken up domestic pigeons.
I have kept every breed which I could purchase or
obtain, and have been most kindly favoured withskins from several quarters of the world, more especially by the Hon. W. Elliot from India, and by the
Hon. C. Murray from Persia. Many treatises in different languages have been published on pigeons, andsome of them are very important, as being of considerable antiquity. I have associated with several
eminent fanciers, and have been permitted to join two

CHAP. I.

DOMESTIC PIGEONS.

21of the London Pigeon Clubs. The diversity of thebreeds is something astonishing. Compare the English
carrier and the shorhfaced tumbler, and see the wonderful difference in their beaks, entailing corresponding
differences in their skulls. The carrier, more especiallythe male bird, is also remarkable from the wonderfuldevelopment of the carunculated skin about the head,
and this is accompanied by greatly elongated eyelids,
very large external orifices to the nostrils, and a wide
gape of mouth. The short-faced tumbler has a beakin outline almost like that of a finch ; and the common
tumbler has the singular and strictly inherited habit offlying at a great height in a compact flock, and tumblingin the air head over heels. The runt is a bird of greatsize, with long, massive beak and large feet; some ofthe sub-breeds of runts have very long necks, others very
long wings and tails, others singularly short tails. The
barb is allied to the carrier, but, instead of a very long
beak, has a very short and very broad one. The pouterhas a much elongated body, wings, and legs; and its
enormously developed crop, which it glories in inflating,
may well excite astonishment and even laughter. The
turbit has a very short and conical beak, with a line ofreversed feathers down the breast; and it has the habitof continually expanding slightly the upper part of thecesophagus. The Jacobin has the feathers so muchreversed along the back of the neck that they forma hood, and it has, proportionally to its size, muchelongated wing and tail feathers. The trumpeter andlaugher, as their names express, utter a very different
coo from the other breeds. The fantail has thirty oreven forty tail-feathers, instead of twelve or fourteen,
the normal number in all members of the great pigeonfamily ; and these feathers are kept expanded, and are
carried so erect that in good birds the head and tail

22

DOMESTIC PIC3EONS.

CHAP. I.

touch; the oil-gland is quite aborted. Several otherless distinct breeds might have been specified.
I n the skeletons of the several breeds, the development of the bones of the face in length and breadth andcurvature differs enormously. The shape, as well as
the breadth and length of the ramus of the lower jaw,
varies in a highly remarkable manner. The number ofthe caudal and sacral vertebm vary ; as does the numberof the ribs, together with their relative breadth and thepresence of processes. The size and shape of theapertures in the sternum are highly variable ; so is thedegree of divergence and relative size of the two arms of
the furcula. The proportional width of the gape of mouth,
the proportional length of the eyelids, of the orifice ofthe nostrils, of the tongue (not always in strict correl&tion with the length of beak), the size of the crop andof the upper part of the esophagus; the development
and abortion of the oil-gland ; the number of the primarywing and caudal feathers ; the relative length of wing
and tail to each other and to the body; the relative
length of leg and of the feet; the number of scutella?
on the toes, the development of skin between the toes,:.Te all points of structure which are variable. The period
at which the perfect plumage is acquired varies, as does
the state of the down with which the nestling birds areclothed when hatched. The shape and size of the eggs
vary. The manner of flight differs remarkably ; as doesin some breeds the voice and disposition. Lastly, incertain breeds, the males and females have come todiffer to a slight degree from each other.
Altogether at least a score of pigeons might bechosen, which if shown to an ornithologist, and he weretold that they were wild birds, would certainly, I think,be ranked by him as well-defined species. Moreover,I do not believe that any ornithologist would place

CHAP. I.

DOMESTIC PIGEONS.

23

the English carrier, the short-faced tumbler, the runt,the barb, pouter, and fantail in the same genus ; more
especially as in each of these breeds several trulyinherited sub-breeds, or species as he might have called
them, could be shown him.
Great as the differences are between the breeds of
pigeons, I am fully convinced that the common opinion ofnaturalists is correct, namely, that all have descended
from the rock-pigeon (Columba livia), including under
this term several geographical races or sub-species, which
differ from each other in the most trifling respects. As
several of the reasons which have led me to this belief
are in some degree applicable in other cases, I will here
briefly give them. If the several breeds are not varieties,
and have not proceeded from the rock-pigeon, they musthave descended from at least seven or eight aboriginal
stocks ; for it is impossible to make the present domestic
breeds by the crossing of any lesser number : how, forinstance, could a pouter be produced by crossing two
breeds unless one of the parent-stocks possessed thecharacteristic enormous crop ? The supposed aboriginal
stocks must all have been rock-pigeons, that is, notbreeding or willingly perching on trees. But besicles
C. livia, with its geographical sub-species, only two or
three other species of rock-pigeons are known ; and thesehave not any of the characters of the domestic breeds.
Hence the supposed aboriginal stocks must either still
exist in the countries where they were originally domesticated, and yet be unknown to ornithologists ; and this,considering their size, habits, and remarkable characters,
seems very improbable; or they must have become
extinct in the wild state. But birds breeding on precipices, and good fliers, are unlikely to be exterminated ;and the commonrock-pigeon, which has the same habitswith the domestic breeds, has not been exterminated24

DOMESTIC PIGEONS.

CHAP.I.

even on several of the smaller British islets, or on the
shores of the Mediterranean. Hence the supposed extermination of so many species having similar habitswith the rock-pigeon seems to me a very rash assump
tion. Moreover, the several above-named domesticatedbreeds have been transported to all parts of the world,
and, therefore, some of them must have been carriedback again into their native country; but not one hasever become wild or feral, though the dovecot-pigeon,
which is the rock-pigeon in a very slightly altered state,
has become feral in several places. Again, all recent
experience shows that it is most difficult to get any wild
animal to breed freely under domestication ; yet on the
hypothesis of the multiple origin of our pigeons, it must
be assumed tha.t at least seven or eight species were so
thoroughly domesticated in ancient times by half-civilized man, as to be quite prolific under confinement,
An argument, as it seems to me, of great weight, andapplicable in several other cases, is, that the abovespecified breeds, though agreeing generally in constitution, habits, voice, colouring, and in most parts of theirstructure, with the wild rock-pigeon, yet are certainly
highly abnormal in other parts of their structure: wemay look in vain throughout the whole great family of
Columbidae for a beak like that of the English carrier,or that of the short-faced tumbler, or barb ; for reversedfeathers like those of the jacobin; for a crop like thatof the pouter ; for tail-feathers like those of the fantail.
Hence it must be assumed not only that half-civilized
man succeeded in thoroughly domesticating several
species, but that he intentionally or by chance pickedout extraordinarily abnormal species ; and further, thatthese very species have since all become extinct or unknown. So many strange contingencies seem to me
improbable in the highest degree.

CHAP. I.

DOMESTIC PIOEONS.

25

Some facts in regard to the colouring of pigeons
well deserve consideration. The rock-pigeon is of aslaty-blue, and has a white rump (the Indian subspecies, C. intermedia of Strickland, having it bluish) ;the tail has a terminal dark bar, with the bases of theouter feathers externally edged with white ; the wings
have two black bars; some semi-domestic breeds andsome apparently truly wild breeds have, besides thetwo black bars, the wings chequered with black. These
several marks do not occur together in any other speciesof the whole family. Now, in every one of the domestic
breeds, taking thoroughly well-bred birds, all the abovemarks, even to the white edging of the outer tailfeathers, sometimes concur perfectly developed. Moreover, when two birds belonging to two distinct breeds
are crossed, neither of which is blue or has any of theabove-specified marks, the mongrel offspring are very
apt suddenly to acquire these characters ; for instance,
I crossed some uniformly white fantails with some
uniformly black barbs, and they produced mottled
brown and black birds ; these I again crossed together,and one grandchild of the pure white fantail and pure
black barb was of as beautiful a blue colour, with thewhite rump, double black wing-bar, and barred andwhite-edged tail-feathers, as any wild rock-pigeon ! We
can understand these facts, on the well-known principleof reversion to ancestral characters, if all the domestic
breeds have descended from the rock-pigeon. But if
we deny this, we must make one of the two following
highly improbable suppositions. Either, fistly, that all
the several imagined aboriginal stocks were coloured
and marked like the rock-pigeon, although no otherexisting species is thus coloured and marked, so that ineach separate breed there might be a tendency to revert
to the very same colours and markings. Or, secondly,

26

DOMESTIC PIGEONS.

CHAP. I.

that each breed, even the purest, has within a dozen or,
at most, within a score of generations, been crossed by
the rock-pigeon : I say within a dozen or twenty generations, for we know of no fact countenancing the belief
that the child ever reverts to some one ancestor, removed
by a greater number of generations. I n a breed which
has been crossed only once with some distinct breed, thetendency to reversion to any character derived from suchcross will naturally become less and less, as in each sdcceeding generation there will be less of the foreign blood ;but when there has been no cross with a distinct breed,
and there is a tendency in both parents to revert to acharacter, which has been lost dining some former generation, this tendency, for all that we can see to thecontrary, may be transmitted undiminished for an indefinite number of generations. These two distinct cases areoften confounded in treatises on inheritance.
Lastly, the hybrids or mongrels from between all thedomestic breeds of pigeons are perfectly fertile. I canstate this from my own observations, purposely madeon the most distinct breeds. Now, it is difhult, perhaps impossible, to bring forward one case of the hybrid
offspring of two animals clearly distinct being themselves
perfectly fertile. Some authors believe that long-continued domestication eliminates this strong tendency tosterility: from the history of the dog I think there issome probability in this hypothesis, if applied to species
closely related together, though it is unsupported by asingle experiment. But t o extend the hypothesis so
far as to suppose that species, aboriginally as distinct
88 carriers, tumblers, pouters, and fantails now are,should yield offspring perfectly fertile, inter 86, seems tome rash in the extreme,
From these several reasons, namely, the improbabilityof man having formerly got seven or eight 8upposed.

CHAP. I.

DOMESTIC PIGEONS.

27

species of pigeons to breed freeIy under domestication ;these supposed species being quite unknown in a wildstate, and their becoming nowhere feral ; these species
having very abnormal characters in certain respects, a8
compared with all other Columbidae, though so like inmost other respects to the rock-pigeon ; the blue colour
and various marks occasionally appearing in all thebreeds, both when kept pure and when crossed; themongrel offspring being perfectly fertile ;-from theseseveral reasons, taken together, I can feel no doubt that
all our domestic breeds have descended from the Columba livia with its geographical sub-species.
In favour of this view, I may add, firstly, that C. livia,
or the rock-pigeon, has been found capable of domestication in Europe and in India ; and that it agrees in habits
and in a great number of points of structure with all
the domestic breeds. Secondly, although an English
carrier or short-faced tumbler differs immensely in certain characters from the rock-pigeon, yet by comparing
the several sub-breeds of these breeds, more especiallythose brought from distant countries, we can make analmost perfect series between %heextremes of structure.
Thirdly, those characters which are mainly distinctive ofeach breed, for instance the wattle and length of beakof the carrier, the shortness of that of the tumbler, andthe number of tail-feathers in the fantail, are in each
breed eminently variable ; and the explanation of thisfact will be obvious when we come to treat of selection.
Fourthly, pigeons have been watched, and tended withthe utmost care, and loved by many people. They havebeen domesticated for thousands of years in several
quarters of the world; the earliest known record of
pigeons is in the Bth Bgyptian dynasty, about 3000 B.c.,
as was pointed out to me by Professor Lepsius; but
Mr. Birch informs me that pigeons are given in a bill

28

DOMESTIC PIGEONS.

CHAP. I.

of fare in the previous dynasty. In the time of the
Romans, as we hear from Pliny, immense prices were
given for pigeons ; “ nay, they are come to this pass, thatthey can reckon up their pedigree and race.” Pigeons
were much valued by Akber Ehan in India, about theyear 1600 ; never less than 20,000 pigeons were takenwith the court. ‘‘The monarchs of Iran and Turan sent
him some very rare birds ;” and, continues the courtly
historian, “ His Majesty by crossing the breeds, which
method wag never practised before, has improved themastonishingly.” About this same period the Dutch werea8 eager about pigeons as were the old Romans. The
paramount importance of these considerations in explaining the immense amount of variation which pigeons
have undergone, will be obvious when we treat of Selection. We shall then, also, see how it is that the breeds
80 often have a somewhat monstrous character. It isalso a most favourable circumstance for the productionof distinct breeds, that male and female pigeons can beeasily mated for life; and thus different breeds can be
kept together in the same aviary.
I have discussed the probable origin of domestic
pigeons at some, yet quite insufficient, length ; because
when I first kept pigeons and watched the several kinds,
knowing well how true they bred, I felt fully as much
dif6culty in believing that they could ever have descended
from a common parent, as any naturalist could in coming
to a similar conclusion in regard to the many species offinches, or other large groups of birds, in nature. One
circumstance has struck me much; namely, that all
the breeders of the various domestic animals and thecultivators of plants, with whom I have ever conversed,
or whose treatises I have read, are 6rmly convincedthat the several breeds to which each has attended, aredescended from so many aboriginally distinct species.

CHAP. I.

SELECTION BY MAN.

29

Ask, as I have asked, a celebrated raiser of Hereford
cattle, whether his cattle might not have descended
from long-horns, and he will laugh you to scorn. I
have never met a pigeon, or poultry, or duck, or
rabbit fancier, who was not fully convinced that eachmain breed was descended from a distinct species. Van
Mons, in his treatise on pears and apples, shows howutterly he disbelieves that the several sorts, for instancea Ribston-pippin or Codlin-apple, could ever have proceeded from the seeds of the same tree. Innumerable
other examples could be given. The explanation, I
think, is simple: from long-continued study they arestrongly impressed with the differences between theseveral races; and though they well know that eachrace varies slightly, for they win their prizes by selecting such slight differences, yet they ignore all generalarguments, and refuse to sum up in their minds slightdifferences accumulated during many successive generations. May not those naturalists who, knowing far lessof the laws of inheritance than does the breeder, andknowing no more than he does of the intermediate links
in the long lines of descent, yet admit that many of ourdomestic races have descended from the same parenbmay they not learn a lesson of caution, when they deride the idea of species in a state of nature being lineal
descendants of other species 3
Xelection.-Let us now briefly consider the steps bywhich domestic races have been produced, either fromone or from several allied species. Some little effect may,perhaps, be attributed to the direct action of the external
conditions of life, and some little to habit ; but he would
be a bold man who would account by such agencies forthe differences of a dray and race horse, a greyhoundand bloodhound, a carrier and tumbler pigeon. One of
the most remarkable features in our domesticated races

30

SELECTION BY MAN.

CHAP. I.

is that we see in them adaptation, not indeed to theanimal’s or plant’s own good, but to man’s use or fancy.
Some variations useful to him have probably arisen
suddenly, or by one step ; many botanists, for instance,
believe that the fuller’s teazle, with its hooks, whichcannot be rivalled by any mechanical contrivance, isonly a variety of the wild Dipsacus ; and this amount ofchange may have suddenly arisen in a seedling. So it
has probably been with the turnspit dog; and this is
known to have been the case with the ancon sheep. But
when we compare the dray-horse and race-horse, thedromedary and camel, the various breeds of sheep fitted
either for cultivated land or mountain pasture, with thewool of one breed good for one purpose, and that ofanother breed for another purpose ; when we compare
the many breeds of dogs, each good for man in very
different ways; when we compare the game-cock, so
pertinacious in battle, with other breeds so little quarrelsome, with everlasting layers ” which never desire tosit, and with the bantam so small and elegant; when
we compare the host of agricultural, culinaiy, orchard,and flower-garden races of plants, most useful to man at
different seasons and for different purposes, or so beautiful in his eyes, we must, I think, look further than tomere variability. We cannot suppose that all the breeds
were suddenly produced as perfect and as useful as we
now see them; indeed, in several cases, we know that
this has not been their history. The key is man’s powerof accumulative selection : nature gives successive variations; man adds them up in certain directions useful tohim. I n this sense he may be said to make for himself
useful breeds.
The great power of this principle of selection is not
hypothetical. It is certain that several of our eminent
breeders have, even within a single lifetime, modified to((CHAP.

I.

SELECTION BY MAN.

31

a large extent some breeds of cattle and sheep. In
order fully to realise what they have done, it is almost
necessary to read several of the many treatises devoted
to this subject, and to inspect the animds. Breeders
habitually speak of an animal’s organisation as something quite plastic, which they can model almost as theyplease. If I had space I could quote numerous passages
to this effect from highly competent authorities. Youatt,
who was probably better acquainted with the works ofagriculturists than almost any other individual, and who
was himself a very good judge of an animal, speaks ofthe principle of selection as “ that which enables the
agriculturist, not only to modify the character of his
flock, but to change it altogether. It is the magician’swand, by means of which he may summon into life whatever form and mould he pleases.” Lord SomervilIe,
speaking of what breeders have done for sheep, says :“It would seem as if they had chalked out upon a walla form perfect in itself, and then had given it existence.”
That most skilful breeder, Sir John Sebright, used tosay, with respect to pigeons, that “he would produce
any given feather in three years, but it would take him
six years to obtain head and beak.” In Saxony the importance d the principle of selection in regard to merino
sheep is so fully recognised, that men follow it as atrade: the sheep are placed on a table and are studied,like a picture by a connoisseur ; this is done three times
at intervals of months, and the sheep are each time
marked and classed, so that the very best may ultimately
be selected for breeding.
What English breeders have actually effected isproved by the enormous prices given for animals with agood pedigree ; and these have now been exported toalmost every quarter of the world. The improvement isby no means generally due to crossing different breeds ;32

SELECTION BY MAN.

CHAP. I.

all the best breeders are strongly opposed to this practice, except sometimes amongst closely allied sub-breeds.
And when a cross has been made, the closest selection is
far more indispensable even than in ordinary cases. If
selection consisted merely in separating some very distinct variety, and breeding from it, the principle would
be so obvious as hardly to be worth notice ; but its importance consists in the great effect produced by theaccumulation in one direction, during successive generations, of differences absolutely inappreciable by an
uneducated eye-differences which I for one have vainly
attempted to appreciate. Not one man in a thousand
has accuracy of eye and judgment sufficient to become
an eminent breeder. If gifted with these qualities, andhe studies his subject for years, and devotes his lifetime
to it with indomitable perseverance, he will succeed, andmay make great improvements ; if he wants any of thesequalities, he will assuyedly fail. Few would readily
believe in the natural capacity and years of practicerequisite to become even a skilful pigeon-fancier.
The same principles are followed by horticulturists ;but the variations are here often more abrupt. No one
supposes that our choicest productions have been produced by a single variation from the aboriginal stock.
We have proofs that this is not so in aome cases, in which
exact records have been kept; thus, to give a very
trifling instance, the steadily-increasing size of the common gooseberry may be quoted. We see an astonishing
improvement in many florists’ flowers, when the flowers of
the present day are compared with drawings made only
twenty or thirty years ago. When a race of plants isonce pretty well established, the seed-raisers do not pickout the best plants, but merely go over their seed-beds,
and pull up the “rogues,” as they call the plants thatdeviate from the proper standard. With animals this

CHAP. I.

METHODICAL SELECTION.

33

kind of selection is, in fact, also followed ; for hardly any
one is so careless as to allow his worst animals to breed.
I n regard to plants, there is another means of observing the accumulated effects of selection-namely, bycomparing the diversity of flowers in the different varieties of the same species in the flower-garden ; the diversityof leaves, pods, or tubers, or whatever part is valued, inthe kitchen-garden,in comparison with the flowers of thesame varieties; and the diversity of fruit of the same
species in the orchard, in comparisonwith the leaves andflowers of the same set of varieties. See how different
the leaves of the cabbage are, and how extremely alikethe flowers ; how unlike the flowers of the heartsease are,and how alike the leaves; how much the fruit of thedifferent kinds of gooseberries differ in size, colour, shape,and hairiness, and yet the flowers present very slightdifferences. It is not that the varieties which differ
largely in some one point do not differ at all in otherpoints; this is hardly ever, perhaps never, the case.
The laws of correlation of growth, the importance of
which should never be overlooked, will ensure some differences; but, as a general rule, I cannot doubt thatthe continued selection of slight variations, either in theleaves, the flowers, or the fruit, will produce races
differing from each other chiefly in these characters.
It may be objected that the principle of selection has
been reduced to methodical practice for scarcely morethan three-quarters of a century ; it has certainly been
more attended to of late years, and many treatises havebeen published on the subject; and the result, I may
add, has been, in a corresponding degree, rapid andimportant. But it is very far from true that the principle is a modern &cove ry. I could give several references to the full acknowledgment of the importance ofthe principle in works of high antiquity. In rude and34

NETHODICAL SELECTION.

CHAP. I.

barbarous periods of English history choice animals were
often imported, and laws were passed to prevent theirexportation: the destruction of horses under a certain
size was ordered, and this may be compared to the‘< roguing ” of plants by nurserymen. The principle of
selection I find distinctly given in an ancient Chinese
encycloptedia. Explicit rules are laid down by some of
the Roman classical writers. From passages in Genesis,
it is clear that the colour of domestic animals was at thatearly period attended to. Savages now sometimes crosstheir dogs with wild canine animals, to improve thebreed, and they formerly did so, as is attested by passagesin Pliny. The savages in South Africa match theirdraught cattle by colour, as do some of the Esquimaux
their teams of dogs. Livingstone shows how much gooddomestic breeds are valued by the negroes of the interior of Africa who have not associated with Europeans.
Some of these facts do not show actual selection, butthey show that the breeding of domestic animals was
carefully attended to in ancient times, and is now
attended to by the lowest savages. It would, indeed,
have been a strange fact, had attention not been paid tobreeding, for the inheritance of good and bad qualities is
so obvious.
At the present time, eminent breeders try by methodical selection, with a distinct object in view, to makea new strain or sub-breed, superior to anything existingin the country. But, for our purpose, a kind of Selection, which may be called Unconscious, and which resultsfrom every one trying to possess and breed from the bestindividual animals, is more important. Thus, a man
who intends keeping pointers naturally tries to get as
good dogs as he can, and afterwards breeds from his own
best dogs, but he has no wish or expectation of permanently altering the breed. Nevertheless I cannot

CHAP. I.

UNCONSCIOUS SELECTION.

35

doubt that this process, continued during centuries,
would improve and modify any breed, in the same way
as Bakewell, Collins, kc., by this very same process,only carried on more methodically, did greatly modify,
even during their own lifetimes, the forms and qualitiesof their cattle. Slow and insensible changes of thiskind could never be recognised unless actual measurements or careful drawings of the breeds in question
had been made long ago, which might serve for comparison. I n some cases, however, unchanged or but
little changed individuals of the same breed may be foundin less civilised districts, where the breed has been lessimproved. There is reason to believe that King Charles’s
spaniel has been unconsciously modified to a large extent
since the time of that monarch. Some highly competentauthorities are convinced that the setter is directly derived
from the spaniel, and has probably been slowly alteredfrom it. It is known that the English pointer has been
greatly changed within the last century, and in thiscase the change has, it is believed, been chiefly effectedby crosses with the fox-hound; but what concerns us
is, that the change has been effected unconsciously andgradually, and yet so effectually, that, though the old
Spanish pointer certainly came from Spain, Mr. Borrow
has not seen, as I am informed by him, any native dog
in Spain like our pointer.
By a similar process of selection, and by careful training, the whole body of English racehorses have come tosurpass in fleetness and size the parent Arab stock, sothat the latter, by the regulations for the Goodwood Races,
are favoured in the weights they carry. Lord Spencer
and others have shown how the cattle of England haveincreased in weight and in early maturity, compared withthe stock formerly kept in this country. By comparing
the accounts given in old pigeon treatises of carriers

36

UNCONSCIOUS SELECTION.

CHAP.I.

and tumblers with these breeds as now existing in
Britain, India, and Persia, we can, I think, clearly trace
the stages through which they have insensibly passed,and come to differ so greatly from the rock-pigeon.
Youatt gives an excellent illustration of the effects ofa course of selection, which may be considered as unconsciously followed, in so far that the breeders could
never have expected or even have wished to have produced the result which ensued-namely, the productionof two distinct strains. The two flocks of Leicester sheep
kept by Mr. Buckley and Mr. Burgess, as Mr. Youatt remarks, " have been purely bred from the original stock of
Mr. Bakewell for upwards of fifty years. There is not a
suspicion existing in the mind of any one at all acquainted with the subject that the owner of either ofthem has deviated in any one instance from the pure
blood of Mr. Bakewell's flock, and yet the difference
between the sheep possessed by these two gentlemen isso great that they have the appearance of being quite
different varieties."
If there exist savages so barbarous as never to thinkof the inherited character of the offspring of theirdomestic animals, yet any one animal particularly usefulto them, for any special purpose, would be carefully
preserved during famines and other accidents, to whichsavages are so liable, and such choice animals would thus
generally leave more offspring than the inferior ones ;so that in this case there would be a kind of unconscious selection going on. We see the yalue set on
animals even by the barbarians of Tierra del Fuego, by
their killing and devouring their old women, in times ofdearth, as of less value than their dogs.
I n plants the same gradual process of improvement,
through the occasional preservation of the best individuals, whether or not sufiiciently distinct to be ranked

CHAP. I.

UNCONSCIOUS SELECTION.

37

at their first appearance as distinct varieties, and whether
or not two or more species or races have become
blended together by crossing, may plainly be recognisedin the increased size and beauty which we now see in thevarieties of the heartsease, rose, pelargonium, dahlia, andother plants, when compared with the older varieties orwith their parent-stocks. No one would ever expect toget a &st-rate heartsease or dahlia from the seed of awild plant. No one would expect to raise a first-rate
melting pear from the seed of the wild pear, though
he might succeed from a poor seedling growing wild,
if it had come from a garden-stock. The pear, thoughcultivated in classical times, appears, from Pliny’s description, to have been a fruit of very inferior quality.
I have seen great surprise expressed in horticultural works
at the wonderful skill of gardeners, in having produced
such splendid results from such poor materials ; but theart, I cannot doubt, has been simple, and, as far as the
final result is concerned, has been followed almost unconsciously. It has consisted in always cultivating thebest known variety, sowing its seeds, and, when a slightly
better variety has chanced to appear, selecting it, andso onwards. But the gardeners of the classical period,
who cultivated the best pear they could procure, neverthought what splendid fruit we should eat; though we
owe our excellent fruit, in some small degree, to theirhaving naturally chosen and preserved the best varieties
they could anywhere find.
A large amount of change in our cultivated plants,
thus slowly and unconsciously accumulated, explains, as
I believe, the well-known fact, that in a vast number ofcases we cannot recognise, and therefore do not know,
the wild parent-stocks of the plants which have been
longest cultivated in our flower and kitchen gardens,
If it has taken centuries or thousands of years to improve

38

SELECTION BY MAN.

CHAP. I.

or modify most of our plants up to their present standardof usefulness to man, we can understand how it is thatneither Australia, the Cape of Good Hope, nor any otherregion inhabited by quite uncivilised man, has afforded USa single plant worth culture. It is not that these countries, so rich in species, do not by a strange chance possess
the aboriginal stocks of any useful plants, but that thenative plants have not been improved by continued selection up to a standard of perfection comparable withthat given to the plants in countries anciently civilised.
I n regard to the domestic animals kept by uncivilisedman, it should not be overlooked that they almost
always have to struggle for their own food, at least
during certain seasons. And in two countries very differently circumstanced, individuals of the same species,
having slightly different constitutions or structure, would
often succeed better in the one country than in theother, and thus by a process of ‘‘natural selection,” aswill hereafter be more fully explained, two sub-breeds
might be formed. This, perhaps, partly explains what
has been remarked by some authors, namely, that thevarieties kept by savages have more of the character of
species than the varieties kept in civilised countries.
On the view here given of the all-important part which
selection by man has played, it becomes at once obvious,
how it is that our domestic races show adaptation in theirstructure or in their habits to man’s wants or fancies.
We can, I think, further understand the frequently
abnormal character of our domestic races, and likewise
their differences being so great in external charactersand relatively so slight in internal parts or organs.
Man can hardly select, or only with much difficulty, any
deviation of structure excepting such as is externallyvisible ; and indeed he rarely cares for what is internal.
He can never act by selection, excepting on variations

CHAP. I.

SELECTION BY MAN.

39

which are first given to him in some slight degree by
nat’ure. No man would ever try to make a fantail, till
he saw a pigeon with a tail developed in some slight degree in an unusual manner, or a pouter till he saw apigeon with a crop of somewhat unusual size ; and themore abnormal or unusual any character was when it &st
appeared, the more likely it would be to catch his attention. But to use such an expression as trying to makea fantail, is, I have no doubt, in most cases, utterly incorrect. The man who first selected a pigeon with aslightly larger tail, never dreamed what the descendantsof that pigeon would become through long-continued,
partly unconscious and partly methodical selection. Perhaps the parent bird of all fantails had only fourteen tailfeathers somewhat expanded, like the present Java fantail,or like individuals of other and distinct breeds, in which
as many as seventeen tail-feathers have been counted.
Perhaps the first pouter-pigeon did not inflate its crop
much more than the turbit now does the upper part ofits esophagus,-a habit which is disregarded by all
fanciers, as it is not one of the points of the breed.
Nor let it be thought that some great deviation of
structure would be necessary to catch the fancier’s eye :he perceives extremely small differences, and it is in
human nature to value any novelty, however slight, in
one’s own possession. Nor must the value which would
formerly be set on any slight differencesin the individualsof the same species, be judged of by the vahe which
would now be set on them, after several breeds haveonce fairly been established. Nany slight differences
might, and indeed do now, arise amongst pigeons, which
me rejected as faults or deviations from the standard ofperfection of each breed. The common goose has not
given rise to any marked varieties ; hence the Thoulouseand the common breed, which differ only in colour, that

40

SELECTION BY MAN.

CHAP.I.

most fleeting of characters, have lately been exhibited
as distinct at our poultry-shows.
I think these views further explain what has sometimes
been noticed-namely, that we know nothing about theorigin or history of any of our domestic breeds. But, infact, a breed, like a dialect of a language, can hardly besaid to have had a definite origin. A man preserves andbreeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his
best animals and thus improves them, and the improved
individuals slowly spread in the immediate neighbourhood. But as yet they will hardly have a distinct name,and from being only slightly valued, their history will
be disregarded. When further improved by the same
slow and gradual process, they will spread more widely,
and will get recognised as something distinct and valuable, and will then probably b t receive a provincial
name. In semi-civilised countries, with little free communication, the spreading and knowledge of any new
sub-breed will be a slow process. As soon as the points ofvalue of the new sub-breed are once fully acknowledged,
the principle, as I have called it, of unconscious selection
will always tend,-perhaps more at one period than at
another, as the breed rises or falls in fashion,-perhaps
more in one district than in another, according to thestate of civilisation of the inhabitants,-slowly to add tothe characteristic features of the breed, whatever they
may be. But the chance will be infinitely small of any
record having been preserved of such slow, varying, andinsensible changes.
I must now say a few words on the circumstances,favourable, or the reverse, to man’s power of selection.
A ]ugh degree of variability is obviously favourable,
as freely giving the materials for selection to workon ; not that mere individual differences are not amply

CHAP.

r.

CIRCUNSTANCES FAVOURABLE TO SELECTION. 41

sufficient, with extreme care, to allow of the accumulationof a large amount of modification in almost any desired
direction. But as variations manifestly useful or pleasing
to man appear only occasionally, the chance of their appearance will be much increased by a large number ofindividuals being kept; and hence this comes to be of
the highest importance to success. On this principle
Marshall has remarked, with respect to the sheep of partsof Yorkshire, that “ as they generally belong to poor
people, and are mostly in small lotu, they never can be
improved.” On the other hand, nurserymen, from raisinglarge stocks of the same plants, are generally far more
successful than amateurs in getting new and valuable
varieties. The keeping of a large number of individuals
of a species in any country requires that the species
should be placed under favourable conditions of life, so
as to breed freely in that country. When the individualsof any species are scanty, all the individuals, whatever
their quality may be, will generally be allowed to breed,
and this will effectually prevent selection. But probably
the most important point of all, is, that the animal orplant should be so highly useful to man, or so muchvalued by him, that the closest attent,ion should be
paid to even the slightest deviation in the qualities or
structure of each individual. Unless such attention
be paid nothing can be effected. I have seen it gravely
remarked, that it was most fortunate that the strawberry began to vary just when gardeners began to attend
closely to this plant. No doubt the strawberry had
always varied since it was cultivated, but the slightvarieties had been neglected. As soon, however, as
gardeners picked out individual plants with slightly
larger, earlier, or better fruit, and raised seedlings fromthem, and again picked out the best seedlings andbred from them, then, there appeared (aided by some

42

SUMMARY ON VARIATION

CHAP. 1.

crossing with distinct species) those many admirable
varieties of the strawberry which have been raised
during the last thirty or forty years.
I n the case of animals with separate sexes, facility
in preventing crosses is an important element of successin the formation of new races,-at least, in a country
which is already stocked with other races. I n this respect enclosure of the land plays a part. Wanderingsavages or the inhabitants of open plains rarely possess
more than one breed of the same species. Pigeons canbe mated for life, and this is a great convenience to the
fancier, for thus many races may be kept true, thoughmingled in the same aviary ; and this circumstance musthave largely favoured the improvement and formationof new breeds. Pigeons, I may add, can be propagatedin great numbers and at a very quick rate, and inferior
birds may be freely rejected, as when killed they servefor food. On the other hand, cats, from their nocturnal
rambling habits, cannot be matched, and, although somuch valued by women and children, we hardly eversee a distinct breed kept u p ; such breeds as we dosometimes see are almost always imported from someother country, often from islands. Although I do not
doubt that some domestic animals vary less than others,
yet the rarity or absence of distinct breeds of the cat,the donkey, peacock, goose, &c., may be attributed inmain part to selection not having been brought into
play: in cats, from the difficulty in pairing them; indonkeys, from only a few being kept by poor people, andlittle attention paid to their breeding ; in peacocks, fromnot being very easily reared and a large stock not kept ;in geese, from being valuable only for two purposes, foodand feathers, and more especially Gom no pleasure having been felt in the display of distinct breeds.

CHAP. I.

UNDER DOMESTICATION.

43

To sum up on the origin of our Domestic Races ofanimals and plants. I believe that the conditions oflife, from their action on the reproductive system, areso far of the highest importance as causing variability.
I do not believe that variability is an inherent andnecessary contingency, under all circumstances, with allorganic beings, as some authors have thought. The
effects of variability are modified by various degrees ofinheritance and of reversion. Variability is governedby many unknown laws, more especially by that of COP
relation of growth. Something may be attributed tothe direct action of the conditions of life. Something
must be attributed to use and disuse. The final result
is thus rendered infinitely complex. In some cases, I
do not doubt that the intercrossing of species, aboriginally distinct, has played an important part in the originof our domestic productions. When in any country
several domestic heeds have once been established,
their occasional intercrossing, with the aid of selection,
has, no doubt, largely aided in the formation of new
sub-breeds ; but the importance of the crossing ofvarieties has, I believe, been greatly exaggerated, bothin regard to aniinals and to those plants which are propagated by seed. I n plants which are temporarily propagated by cuttings, buds, &c., the importance of thecrossing both of distinct species and of varieties is immense ; for the cultivator here quite disregards the extreme variability both of hybrids and mongrels, and thefrequent sterility of hybrids ; but the cases of plants notpropagated by seed are of little importance to us, fortheir endurance is only temporary, Over all thesecauses of Change I am convinced that the accumulative action of Selection, whether applied methodicallyand more quickly, or unconsciously and more slowly,
but more efficiently, is by far the predominant Power.

44

VARIATION UNDER NATURE.

CHAP. 11.

C H A P T E R 11.
VARIATION
UNDER NATURE.
Variability - Individual differences - Doubtful species - Wideranging, much diffused, and common species vary m o s t S p e cies of the larger genera in any country vary more than the speciesof the smaller genera-Many of the species of the larger genera
resemble varieties in being very closely, but unequally, related
to each other, and in having restricted ranges.

BEFORE
applying the principles arrived at in the lastchapter to organic beings in a state of nature, we must
briefly discuss whether these latter are subject to any
variation. To treat this subject at all properly, a longcatalogue of dry facts should be given ; but these I shallreserve for my future work. Nor shall I here discuss
the various definitions which have been given of theterm species. No one definition has as yet satisfied all
naturalists; yet every naturalist knows vaguely what
he means when he speaks of a species. Generally theterm includes the unknown element of a distinct act of
creation. The term “ variety ” is almost equally diEicult
to define; but here community of descent is almost
universally implied, though it can rarely be proved.
We have also what are called monstrosities ; but they
graduate into varieties. By a monstrosity I presume ismeant some considerable deviation of structure in one
part, either injurious to or not useful to the species, andnot generally propagated. Some authors use the term
“variation ’) in a technical sense, as implying a modification directly due to the physical conditions of life ; and“va.riations)’ in this sense are supposed not to be inherited: but who can say that the dwarfed condition ofshells in the brackish waters of the Baltic, or dwarfed

CHAP.11.

VARIATION UNDER NATURE.

45

plants on Alpine summits, or the thicker fur of ananimal from far northwards, would not in some cases be
inherited for at least some few generations? and in thiscase I presume that the form would be called a variety.
Again, we have many slight differences which may becalled individual differences, such as are known frequently to appear in the offspring from the same parents,
or which may be presumed to have thus arisen, frombeing frequently observed in the individuals of the same
species inhabiting the same confined locality. No one
supposes that all the individuals of the same species arecast in the very same mould. These individual differences are highly important for us, as they afford materials for natural selection to accumulate, in the samemanner as man can accumulate in any given direction
individual differences in his domesticated productions.
These individual differences generally affect what naturalists consider unimportant parts ; but I could show bya long catalogue of facts, that paits which must be called
important, whether viewed under a physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am convinced that the mostexperienced naturalist would be surprised at the numberof the cases of variability, even in important parts ofstructure, which he could collect on good authority, as I
have collected, during a course of years. I t should be
remembered that systematists are far from pleased at
finding variability in important characters, and that there
are not many men who will laboriously examine internal
and important organs, and compare them in many specimens of the same sbecies. I should never have expectedthat the branching of the main nerves close to the great
central ganglion of an insect would have been variablein the same species; I should have expected thatchanges of this nature could have been effected only

46

VARIATION UNDER NATURE.

CEAP. 11.

by slow degrees: yet quite recently Mr. Lubbock has
shown a degree of variability in these main nerves in
Coccus, which may almost be compared to the irregularbranching of the stem of a tree. This philosophicalnaturalist, I may add, has also quite recently shownthat the muscles in the lams of certain insects arevery far from uniform. Authors sometimes argue ina circle when they state that important organs nevervary ; for these same authors practically rank that character as important (as some few naturalists have honestly
confessed) which does not vary; and, under this pointof view, no instance of an important part varying will
ever be found : but under any other point of view many
instances assuredly can be given.
There is one point connected with individual differences, tvhich seems to me extremely perplexing: I
refer to those genera which have sometimes been called“protean” or “polymorphic,” in which the species present
an inordinate amount of variation ; and hardly two naturalists can agree which forms to rank as species andwhich as varieties. We may instance Rubus, Rosa, andHieracium amongst plants, several genera of insects, andseveral genera of Brachiopod shells. In most polymorphic
genera some of the species have fixed and d e h i t e characters. Genera which are polymorphic in one country
seem to be, with some few exceptions, polymorphic inother countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts seem tobe very perplexing, for they seem to show that this kindof variability is independent of the conditions of life.
I am inclined to suspect that we see in these polyniorphic genera variations in points of structure which areof no service or disservice to the species, and which consequently have not been seized on and rendered defmite
by natural selection, as hereafter w i l l be explained,CHAP. 11.

DOUBTFUL SPECIES.

47

Those forms which possess in some considerable
degree the character of species, but which are so closely
similar to some other forms, or are so closely linked tothem by intermediate gradations, that naturalists do not
like to rank them as distinct species, are in several respects the most important for us. We have every reason
to believe that many of these doubtful and closely-allied
forms have permanently retained their characters intheir own country for a long time ; for as long, as far
as we know, as have good and true species. Practically, when a naturalist can unite two forms together
by others having intermediate characters, he treats theone as a variety of the other, ranking the most common,but sometimes the one first described, as the species,
and the other as the variety. But cases of great d B culty, which I will not here enumerate, sometimes
occur in deciding whether or not to rank one form asa variety of another, even when they are closely connected by intermediate links ; nor will the commonlyassumed hybrid nature of the intermediate links always
remove the difficulty. In very many cases, however,
one form is ranked as avariety of another, not because
the intermediate links have actually been found, but
because analogy leads the observer to suppose either
that they do now somewhere exist, or may formerly
have existed; and here a wide door for the entry of
doubt and conjecture is opened.
Hence, in determining whether a form should be
ranked as a species or a variety, the opinion of naturalists having sound judgment and wide experience seems
the only guide to follow. We must, however, in manycases, decide by a majority of naturalists, for few wellmarked and well-known varieties can be named whichhave not been ranked as species by at least some competent judges.

48

DOUBTFUL SPECIES.

C H a p . 11.

That varieties of this doubtful nature are far fromuncommon cannot be disputed. Compare the several
floras of Great Britain, of France or of the UnitedStates, drawn up by different botanists, and see what asurprising number of forms have been ranked by one
botanist as good species, and by another as mere
varieties. Mr. H. C. Watson, to whom I lie underdeep obligation for assistance of all kinds, has marked
for me 182 British plants, which are generally considered as varieties, but which have all been ranked
by botanists as species; and in making this list hehas omitted many trifling varietiee, but which nevertheless have been ranked by some botanists as species,
and he has entirely omitted several highly polymorphic
genera, Under genera, including the most polymorphic
forms, Mr. Babington gives 251 species, whereas Mr.
Bentham gives only 112,-a difference of 139 doubtful
forms1 Amongst animals which unite for each birth,and which are highly locomotive, doubtful forms, ranked
by one zoologist as a species and by another as a variety,can rarely be found within the same country, but arecommon in separated areas. How many of those birds
and insects in North America and Europe, which differ
very slightly from each other, have been ranked by
one eminent naturalist as undoubted species, and byanother as varieties, or, as they are often called, as
geographical races ! Many years ago, when comparing,and seeing others compare, the birds from the separate islands of the Galapagos Archipelago, both one
with another, and with those from the American mainland, I was much struck how entirely vague and arbitrary is the distinction between species and varieties.
On the islets of the little Madeira group there aremany insects which are characterized as varieties in
Nr.Wollaston’s admirable work, but which it cannot

CHAP. 11.

DOUBTFUL SPECIES.

49

be doubted would be ranked as distinct species by many
entomologists. Even Ireland has a few animals, now
generally regarded as varieties, but which have beenranked as species by some zoologists. Several mostexperienced ornithologists consider our British redgrouse as only a strongly-marked race of a Norwegianspecies, whereas the greater number rank it as an
undoubted species peculiar to Great Britain. A wide
distance between the homes of two doubtful forms leads
many naturalists to rank both as distinct species ; butwhat distance, it has been well asked, will suffice? ifthat between America and Europe is ample, w i l l thatbetween the Continent and the Azores, or Madeira, or
the Canaries, or Ireland, be sufficient? It must be
admitted that many forms, considered by highly-competent judges as varieties, have so perfectly the characterof species that they are ranked by other highly competent judges as good and true species. But to discuss
whether they are rightly called species or varieties,
before any definition of these terms has been generally
accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or
doubtful species well deserve consideration ; for several
interesting lines of argument, from geographical distribution, analogical variation, hybridism, &c., havebeen brought to bear on the attempt to determine theirrank. I will here give only a single instance,-thewell-known one of the primrose and cowslip, or Primulaveris and elatior. These plants differ considerably inappearance; they have a different flavour and emit adifferent odour ; they flower at slightly different periode ;they grow in somewhat different stations ; they ascend
mountains to different heights ; they have different
geographical ranges ; and lastly, according to very
numerous experiments made during several years by

50

DOUBTFUL SPECIES.

CHAP. 11.

that most careful observer Gartner, they can be crossed
only with much difficulty. We could hardly wish forbetter evidence of the two forms being specifically distinct. On the other hand, they are united by many
intermediate links, and it is very doubtful whether theselinks are hybrids; and there is, as it seems to me, anoverwhelming amount of experimental evidence, showing that they descend from common parents, and consequently must be ranked as varieties.
Close investigation, in most cases, will bring naturalists
to an agreement how to rank doubtful forms. Yet it
must be confessed, that it is in the best-known countries
that we find the greatest number of forms of doubtful
value. I have been struck with the fact, that if any
animal or plant in a state of nature be highly useful
to man, or from any cause closely attract his attention,varieties of it will almost universally be found recorded.
These varieties, moreover, will be often ranked by someauthors as species. Look at the common oak, howclosely it has been studied; yet a German author
makes more than a dozen species out of forms, which
are very generally considered as varieties ; and in thiscountry the highest botanical authorities and practicalmen can be quoted to show that the sessile and pedunculated oaks are either good and distinct species ormere varieties.
When a young naturalist commences the study of agroup of organisms quite unknown to him, he is at firstmuch perplexed to determine what differences to consider
as specific, and what as varieties ; for he knows nothingof the amount and kind of variation to which the groupis subject ; and this shows, at least, how very generally
there is some variation. But if he confine his attention
to one class within one country, he will soon make up
his mind how to rank most of the doubtful forms. His

CIIAP. 11.

DOUBTFUL SPECIES.

51

general tendency will be to make many species, for hewill become impressed, just like the pigeon or poultryfancier before alluded to, with the amount of differencein the forms which he is continually studying; and hehas little general knowledge of analogical variation inother groups and in other countries, by which to correct
his first impressions. As he extends the range of his
observations, he will meet with more cases of difficulty ;for he will encounter a greater number of closely-allied
forms. But if his observations be widely extended, hewill in the end generally be enabled to make up his own
mind which to call varieties and which species ; but hewill succeed in this at the expense of admitting muchvariation,-and the truth of this admission will often be
disputed by other naturalists. When, moreover, hecomes to study allied forms brought from countries not
now continuous, in which case he can hardly hope tofind the intermediate links between his doubtful forms,
he will have to trust almost entirely to analogy, and his
difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet
been drawn between species and sub-species-that is,
the forms which in the opinion of some naturalists comevery near to, but do not quite arrive at the rank ofspecies ; or, again, between sub-species and well-marked
varieties, or between lesser varieties and individual differences. These differences blend into each other in an
insensible series ; and a series impresses the mind with
the idea of an actual passage.
Hence I look at individual differences, though ofsmall interest to the systematist, as of high importancefor us, as being the first step towards such slight varieties
as are barely thought worth recording in works on natural
history. And I look at varieties which are in any degree
more distinct and permanent, as steps leading to more

52

VARIETIES GRADUATE INTO SPECIES.

CHAP.11.

strongly marked and more permanent varieties ; and atthese latter, as leading to sub-species, and to species.
The passage from one stage of difference to another andhigher stage may be, in some cases, due merely to thelong-continued action of different physical conditions in
two different regions; but I have not much faith in
this view ; and I attribute the passage of a variety, froma state in which it differs very slightly from its parent
to one in which it differs more, to the action of natural
selection in accumulating (as will hereafter be more fully
explained) differences of structure in certain definite
directions. Hence I believe a well-marked variety may bejustly called an incipient species ; but whether this belief
be justifiable must be judged of by the general weight ofthe several facts and views given throughout this work.
It need not be supposed that all varieties or incipient
species necessarily attain the rank of species. They may
whilst in this incipient state become extinct, or they may
endure as varieties for very long periods, as has beenshown to be the case by Mr. Wollaston with the varieties of certain fossil land-shells in Nadeira. If a variety
were to flourish so as to exceed in numbers the parent
species, it would then rank as the species, and the species as the variety; or it might come to supplant andexterminate the parent species ; or both might co-exist,
and both rank as independent species. But we shallhereafter have to return to this subject.
From these remarks it will be seen that I look at theterm species, as one arbitrarily given for the sake of conyenience to a set of individuals closely resembling eachother, and that it does not essentially differ from theterm variety, which is given to less distinct and more
fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied
arbitrarily, and for mere convenience sake.

CHAP. 11.

DOMINANT SPECIES VARY NOST.

53

Guided by theoretical considerations, I thought thatsome interesting results might be obtained in regard to
the nature and relations of the species which vary most,by tabulating all the varieties in several well-worked
floras. At first this seemed a simple task ; but Mr. H.
C. Watson, to whom I am much indebted for valuableadvice and assistance on this subject, soon convincedme that there were many difficulties, as did subsequently Dr. Hooker, even in stronger terms. I shallreserve for my future work the discussion of these difficulties, and the tables themselves of the proportional
numbers of the varying species. Dr. Hooker permits
me to add, that after having carefully read my manuscript, and examined the tables, he thinks that the following statements are fairly well established. The wholesubject, however, treated as it necessarily here is withmuch brevity, is rather perplexing, and allusions cannot
be avoided to the “ struggle for existence,” “ divergenceof character,” and other questions, hereafter to be discussed.
Alph. De Candolle and others have shown that plants
which have very wide ranges generally present varieties ;and this might have been expected, as they become exposed to diverse physical conditions, and as they comeinto competition (which, as we shall hereafter see, is a
far more important circumstance) with different sets oforganic beings. But my tables further show that, in
any limited country, the species which are most common,that is abound most in individuals, and the species which
are most widely diffused within their own country (andthis is a different consideration from wide range, and toa certain extent from commonness), often give rise tovarieties sficiently well-marked to have been recorded
in botanical works. Hence it is the most flourishing,or, as they may be called, the dominant species,-

54

DONINANT SPECIES VARY MOST.

CHAP. 11.

those which range widely over the world, are the mostcliffused in their own country, and are the most numerous
in individuals,-which
oftenest produce well-marked
varieties, or, as I consider them, incipient species. And
this, perhaps, might have been anticipated; for, as
varieties, in order to become in any degree permanent,
necessarily have to struggle with the other inhabitantsof the country, the species which are already dominant
will be the most likely to yield offspring which, though
in some slight degree modified, will still inherit thoseadvantages that enabled their parents to become dominant over their compatriots.
If the plants inhabiting a country and described in
any Flora be divided into two equal masses, all those in
the larger genera being placed on one side, and all thosein the smaller genera on the other side, a somewhat
larger number of the very common and much diffused or
dominant species will be found on the side of the larger
genera. This, again, might have been anticipated ; forthe mere fact of many species of the same genus inhabiting any country, shows thai there is something in
the organic or inorganic conditions of that countiyfavourable to the genus ; and, consequently, we might
have expected to have found in the larger genera, or
those including many species, a large proportional numEcr of dominant species. But so many causes tend to
obscure this result, that I am surprised that my tables
show even a small majority on the side of the larger
genera. I will here allude to only two causes ofobscurity. Fresh-water and salt-loving plants havegenerally very wide ranges and are much diffused,
but this seems to be connected with the nature of thestations inhabited by them, and has little or no relation
to the size of the genera to which the species belong.
Again, plants low in the scale of organisation are

OIAP.II.

SPECIES OF LARGE GENERA VARIABLE.

55

generally much more widely diffused than plants higher
in the scale ; and here again there is no close relation
to the size of the genera. The cause of lowly-organised
plants ranging widely will be discussed in our chapter
on geographical distribution.
From looking at species as only strongly-marked andwell-defined varieties, I was led to anticipate that the
species of the larger genera in each country would oftenerpresent varieties, than the species of the smaller genera ;for wherever many closely related species (i. e. species ofthe same genus) have been formed, many varieties orincipient species ought, as a general rule, to be now
forming. Where many large trees grow, we expect tofind saplings. Where many species of a genus havebeen formed through variation, circumstances have beenfavourable for variation; and hence we might expectthat the circumstances would generally be still favourable to variation. On the other hand, if we look at
each species as a special act of creation, there is noapparent reason why more varieties should occur in agroup having many species, than in one having few.
To test the truth of this anticipation I have arranged
the plants of twelve countries, and the coleopterous
insects of two districts, into two nearly equal masses, thespecies of the larger genera on one side, and those of thesmaller genera on the other side, and it has invariably
proved to be the case that a larger proportion of thespecies on the side of the larger genera present varieties,
than on the side of the smaller genera. Moreover, thespecies of the large genera which present any varieties,
invariably present a larger average number of varieties
than do the species of the small genera. Both theseresults follow when another division is made, and when
all the smallest genera, with from only one to four species, are absolutely excluded from the tables. These

56

SPECIES OF LARGE GENERA

CHAP. 11.

facts are of plain signification on the view that species
are only strongly marked and permanent varieties ; forwherever many species of the same genus have beeii
formed, or where, if we may use the expression, themanufactory of species has been active, we ought generally to find the manufactory still in action, more especially as we have every reason to believe the process ofmanufacturing new species to be a slow one. And thiscertainly is the case, if varieties be looked at as incipient species ; €or my tables clearly show as a generalrule that, wherever many species of a genus have been
formed, the species of that genus present a number ofvarieties, that is of incipient species, beyond the average.
It is not that all large genera are now varying much, andare thus increasing in the number of their species, or thatno small genera are now varying and increasing ; for if
this had been so, it would have been fatal to my theory ;inasmuch as geology plainly tells us that small genera
have in the lapse of time often increased greatly insize; and that large genera have often come to theirmaxima, declined, and disappeared. All that we want
to show is, that where many species of a genus havebeen formed, on an average many are still forming;and this holds good.
There are other relations between the species of large
genera and their recorded varieties which deserve notice.
We have seen that there is no infallible criterion bywhich to distinguish species and well-marked varieties ;a;nd in those cases in which intermediate links have notbeen found between doubtful forms, naturalists are compelled to come to a determination by the amount ofdifference between them, judging by analogy whether
or not the amount suflices to raise one or both to therank of species. Hence the amouut of difference is onevery important criterion in settling whether two foirns

CH.4P. 11.

RESEMBLE VARIETIES.

57

should be ranked as species or varieties. Nom Fries
has remarked in regard to plants, and Westwood inregard to insects, that in large genera the amount ofdifference between the species is often exceedingly small.
I have endeavoured to test this numerically by averages,
and, as far as my imperfect results go, they alu-ays confirm the view. I have also consulted some sagacious
and most experienced observers, and, after deliberation,
they concur in this view. I n this respect, therefore, thespecies of the larger genera resemble varieties, morethan do the species of the smaller genera. Or the casemay be put in another way, and it may be said, thatin the larger genera, in which a number of varieties orincipient species greater than the average are now
manufacturing, many of the species already manufactiired still to a certain extent resemble varieties, forthey differ from each other by a less than usual amountof difference.
Moreover, the species of the large genera are related
to each other, in the same manner as fLe varieties ofany one species are related to each other. No naturalist pretends that all the species of a genus are equally
distinct from each other ; they may generally be divided
into sub-genera, or sections, or lesser groups. As Fries
has well remarked, little groups of species are generally
clustered like satellites around certain other species. And
what are varieties but groups of forms, unequally related
to each other, and clustered round certain forms-that is,round their parent-species ? Undoubtedly there is one
most important point of difference between varieties andspecies ; namely, that the amount of difference between
varieties, when compared with each other or with theirparent-species, is much less than that between the species of the same genus. But when we come to discuss
the principle, as I call it, of Divergence of Character,

58

SPECIES OF LARGE GENERA

CHAP.11.

we shall see how this may be explained, and how thelesser differences between varieties will tend to increase
into the greater differences between species.
There is one other point which seems to me worth
notice. Varieties generally have miich restricted ranges :this statement is indeed scarcely more than a truism,
for if a variety were found to have a wider range thanthat of its supposed parent-species, their denominations
ought to be reversed. But there is also reason to believe,that those species wliich are very closely allied toother species, and in so far resemble varieties, often
have much restricted ranges. For instance, Mr. H. C.
Watson has marked for me in the well-sifted London
Catalogue of plants (4th edition) 63 plants which are
therein ranked as species, but which he considers as so
closely allied to other species as to be of doubtful value :these 63 reputed species range on an average over 6.9of the provinces into which Mr. Watson has divided
Great Britain. Now, in this same catalogue, 53 acknowledged varieties are recorded, and these range over 7.7
provinces ; whereas, the species to which these varieties
belong range over 14.3 provinces. So that the acknowledged varieties have very nearly the same restricted
average range, as have those very closely allied forms,
marked for me by Mr. Watson as doubtful species, butwhich are almost universally ranked by British botanists
as good and true species.
Finally, then, varieties have the same general characters as species, for they cannot be distinguished from
species,-except, firstly, by the discovery of intermediate
linking forms, and the occurrence of such links cannot
affect the actual characters of the forms which they
connect; and except, secondly, Ly a certain amount ofCHAP. 11.

RESEMBLE VARIETIES.

59

difference, for two forms, if differing very little, aregenerally ranked as varieties, notwithstanding that intermediate linking forms have not been discovered; but
the amount of difference considered necessary to gire
to two forms the rank of species is quite indefinite. I n
genera having more than the average number of species
in any country, the species of these genera haIre morethan the average number of varieties. In large genera
the species are apt to be closely, but unequally, allied
together, forming little clusters round certain species.
Species very closely allied to other species apparently
have restricted ranges. I n all these several respects the
species of large genera present a strong analogy withvarieties. And we can clearly understand these analogies, if species have once existed as varieties, and havethus originated : whereas, these analogies are utterly
inexplicable if each species has been independently
created.
We have, also, seen that it is the most flourishingand dominant species of the larger genera which on anaverage vary most; and varieties, as we shall hereaftersee, tend to become converted into new and distinct
species. The larger genera thus tend to become larger ;and throughout nature the forms of life which are now
dominant tend to become still more dominant by leaving many modified and dominant descendants. Cut by
steps hereafter to be explained, the larger genera also
tend to break up into smaller genera. And thus, tne
forms of life throughout the universe become divided
into groups subordinate to groups.

60

STRUGGLE FOR EXISTENCE.

CHAP. 111.

C H A P T E R 111.
STRUGGLE
FOR EXISTEPI’CE.
Bears on natural selection-The term used in a wide sense-Geometrical powers of increase
Rapid increase of naturalised
animals and plants-Nature of the checks to increase-Competition universal - Effects of climate - Protection from thenumber of individuals-Complex relations of all animals andplants throughout nature-Struggle for life most severe between
individuals and varieties of the same species ; often severe between species of the same genus-The relation of organism toorganism the most important of all relations.

-

BEFORE
entering on the subject of this chapter, I must
make a few preliminary remarks, to show how the
struggle for existence bears on Natural Selection. It
has been seen in the last chapter that amongst organic
beings in a state of nature there is some individual variability ; indeed I am not aware that this has ever beendisputed. It is immaterial for us whether a multitudeof doubtful forms be called species or sub-species or varieties ; what rank, for instance, the two or three hundred
doubtful forms of British plants are entitled to hold, if
the existence of any well-marked varieties be admitted.
But the mere existence of individual variability and ofsome few well-marked varieties, though necessary as
the foundation for the work, helps us but little inunderstanding how species arise in nature. How have
all those exquisite adaptations of one part of the organisation to another part, and to the conditions of life, andof one distinct organic being to another being, been pcrfected ? We see these beautiful co-adaptations mostplainly in the woodpecker and missletoe; and only alittle less plainly in the humblest parasite which clings

CHAP. 111.

STRUGGLE FOR EXISTENCE.

61

to the hairs of a quadruped or feathers of a bird ; in the
structure of the beetle which dives through the water ;in the plumed seed which is wafted by the gentlest
breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which
I have called incipient species, become ultimately converted into good and distinct species, which in mostcases obviously differ from each other far more than dothe varieties of the same species? How do those groupsof species, which constitute what are called distinct
genera, and which differ from each other more than dothe species of the same genus, arise? All these results,
as we shall more fully see in the next chapter, follow
inevitably from the struwde for life. Owing to thisa9
struggle for life, any variation, however slight and from
whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely
complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring.
The offspring, also, will thus have a better chance ofsurviving, for, of the many individuals of any species
which are periodically born, but a small number cansurvive. I have called this principle, by which eachslight variation, if useful, is preserved, by the term of
Natural Selection, in order to mark its relation to man’s
power of selection. We have seen that man by selecLion can certainly produce great results, and can adapt
organic beings to his own uses, through the accumulation of slight but useful variations, given to him by thehand of Nature. But Natural Selection, as we shallhereafter see, is a power incessantly ready for action,and is as immeasurably superior to man’s feeble efforts,
as the works of Kature are to those of Art.

62

STRUGGLE FOR EXISTENCE.

CHAP. 111.

We will now discuss in a little more detail the struggle
for existence. I n my future work this subject shall betreated, as it well deserves, at much greater length.
The elder De Candolle and Lyell have largely and philosophically shown that all organic beings are exposed
to severe competition. I n regard to plants, no one hastreated this subject with more spirit and ability than
W. Herbert, Dean of Manchester, evidently the resultof his great horticultural knowledge. Nothing is easierthan to admit in words the truth of the universal
struggle for life, or more difTicult-at least I have foundit so-than constantly to bear this conclusion in aind.
Yet unless it be thoroughly engrained in the mind, I
am convinced that the whole economy of nature, withevery fact on distribution, rarity, abundance, extinction,and variation, will be dimly seen or quite misunderstood.
We behold the face of nature bright with gladness, we
often see superabundance of food ; we do not see, or weforget, that the birds which are idly singing round us
mostly live on insects or seeds, and are thus constantly
destroying life ; or we forget how largely these songsters,or their eggs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind,
that though food may be now superabundant, it is not
so at all seasons of each recurring year.
I should premise that I use the term Struggle for
Existence in a large and metaphorical sense, including
dependence of one being on another, and including(which is more important) not only the life of the individual, but success in leaving progeny. Two canine
animals in a time of dearth, may be truly said tostruggle with each other which shall get food and live.
But a plant on the edge of a desert is said to struggle
for life against the drought, though more properly it
should be said to be dependent on the moisture. A

CHAP.111.

STRUGGLE FOR EXISTENCE.

63

plant which annually produces a thousand seeds, ofwhich on an average only one comes to maturity, may
be more truly said to struggle with the plants of thesame and other kinds which already clothe the ground.
The missletoe is dependent on the apple and a few othertrees, but can only in a far-fetched sense be said tostruggle with these trees, for if too many of these parasites grow on the same tree, it will languish and die.
But several seedling missletoes, growing close together
on the same branch, may more truly be said to struggle
with each other. As the missletoe is disseminated bybirds, its existence depends on birds ; and it may metaphorically be said to struggle with other fruit-bearing
plants, in order to tempt birds to devour and thus
disseminate its seeds rather than those of other plants.
In these several senses, which pass into each other, I
use for convenience sake the general term of struggle
for existence.
A struggle for existence inevitably follows from thehigh rate at which all organic beings tend to increase.
Every being, which during its natural lifetime produces
several eggs or seeds, must suffer destruction during someperiod of its life, and during some season or occasional
year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately greatthat no country could support the product. Hence, as
more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same
species, or with the individuals of distinct species, orwith the physical conditions of life. It is the doctrineof Malthus applied with manifold force to the wholeanimal and vegetable kingdoms ; for in this case there
can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may

64

HIGH RATE OF INCREASE.

CHAP. 111.

be now increasing, more or less rapidly, in numbers, allcannot do so, for the world would not hold them.
There is no exception to the rule that every organic
being naturally increases at so high a rate, that if not
destroyed, the earth would soon be covered by the progenyof a single pair. Even slow-breeding man has doubled
in twenty-five years, and at this rate, in a few thousand
years, there would literally not be standing room for hisprogeny. Linnzus has calculated that if an annual
plant produced only two seeds-and there is no plant so
unproductive as this-and their seedlings next year produced two, and so on, then in twenty years there would
be a million plants. The elephant is reckoned to be theslowest breeder of all known animals, and I have takensome pains to estimate its probable minimum rate ofnatural increase : it will be under the mark to assume
that it breeds when thirty years old, and goes on breedingtill ninety years old, bringing forth three pair of young
in this interval; if this be sot at the end of the fifth
century there would be alive fifteen million elephants,
descended from the first pair.
But we have better evidence on this subject thanmere theoretical calculations, namely, the numerous
recorded cases of the astonishingly rapid increase ofvarious animals in a state of nature, when circumstances
have been favourablc to them during two or three following seasons. Still more striking is the evidence fromour domestic animals of many kinds which have run
wild in several parts of the world: if the statements of
the rate of increase of slow-breeding cattle and horses
in South-America, and latterly in Australia, had notbeen well authenticated, they would have been quite
incredible. So it is with plants : cases could be given of
introduced plants which have become common throughout
whole islands in a period of less than ten years. Several

CNAP. 111.

HIGH RATE O F INCREASE.

65

of the plants now most numerousover the wide plains of
La Plata, clothing square leagues of surface almost to
the exclusion of all other plants, have been introduced
from Europe ; and there are plants which now range inIndia, as I hear from Dr. Falconer, from Cape Coniorin
to the Himalaya, which have been imported from America
since its discovery. I n such cases, and endless instances
could be given, no one supposes that the fertility ofthese animals or plants has been suddenly and temporarily increased in any sensible degree. The obvious
explanation is that the conditions of life have been very
favourable, and that there has consequently been lessdestruction of the old and young, and that nearly all the
young have been enabled to breed. I n such cases thegeometrical ratio of increase, the result of which neverfails to be surprising, simply explains the extraordinarily
rapid increase and wide diffusion of naturalised productions in their new homes.
I n a state of nature almost every plant produces seed,
and amongst animals there are very few which do notannually pair. Hence we may confidently assert, thatall plants and animals are tending to increase at a geometrical ratio, that all would most rapidly stock every
station in which they could any how exist, and that thegeometrical tendency to increase must be checked bydestruction at some period of life. Our familiarity withthe larger domestic animals tends, I think, to mislead
us : we see no great destruction falling on them, and weforget that thousands are annually slaughtered for food,and that in a state of nature an equal number would
have somehow to be disposed of.
The only difference between organisms which annually
produce eggs or seeds by the thousand, and those which
produce extremely few, is, that the slow-breeders would
require a few more years to people, under favourable

66

HIGH RATE OF INCREASE.

CHAP. 111.

conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, andyet in the same country the condor may be the more
numerous of the two-3he Fulmar petrel lays but one
egg, yet it is believed to be the most numerous birdin the world. One fly deposits hundreds of eggs, andanother, like the hippobosca, a single one; but thisdifference does not determine how many individuals of
the two species can be supported in a district. A large
number of eggs is of some importance to those species,
which depend on a rapidly fluctuating amount of food,for it allows them rapidly to increase in number. But
the real importance of a large number of eggs or seeds
is to make up for much destruction at some period oflife ; and this period in the great majority of cases is anearly one. If an animal can in any way protect its own
eggs or young, a small number may be produced, andyet the average stock be fully kept up; but if many
eggs or young are destroyed, many must be produced,
or the species will become extinct. It would suffice tokeep up the full number of a tree, which lived on anaverage for a thousand years, if a single seed were produced once in a thousand years, supposing that this seed
were never destroyed, and could be ensured to germinate in a fitting place. So that in all cases, the average
number of any animal or plant depends only indirectlyon the number of its eggs or seeds.
I n looking at Nature, it is most necessary to keep
the foregoing considerations always in mind-never toforget that every single organic being around US may
be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of
its life; that heavy destruction inevitably falls either
on the young or old, during each generation or atrecurrent intervals. Lighten any check, mitigate theCHAP. 111.

CHECKS TO INCREASE.

67

destruction ever so little, and the number of the species
will almost instantaneously increase to any amount.
The face of Nature may be compared to a yielding
surface, with ten thousand sharp wedges packed close
together and driven inwards by incessant blows, sometimes one wedge being struck, and then another withgreater force.
What checks the natural tendency of each species toincrease in number is most obscure. Look at the mostvigorous species ; by as much as it swarms in numbers, by
so much will its tendency to increase be still further increased. We know not exactly what the checks are ineven one single instance. Nor will this surprise any one
who reflects how ignorant we are on this head, even in
regard to mankind, so incomparably better known thanany other animal. This subject has been ably treated byseveral authors, and I shall, in my future work, discuss
some of the checks at considerable length, more especiallyin regard to the feral animals of South America. HereI will make only a few remarks, just to recall to thereader’s mind some of the chief points. Eggs or very
young animals seem generally to suffer most, but thisis not invariably the case. With plants there is a vast
destruction of seeds, but, from some observations which
I have made, I believe that it is the seedlings whichsuffer most from germinating in ground already thickly
stocked with other plants. Seedlings, also, are destroyedin vast numbers by various enemies ; for instance, on apiece of ground three feet long and two wide, dug andcleared, and where there could be no choking from otherplants, I marked all the seedlings of our native weeds as
they came up, and out of the 357 no less than 295 weredestroyed, chiefly by slugs and insect,s. If turf which
has long been mown, and the case would be the samewith turf closeIy browsed by quadrupeds, be Iet to grow,

6s

CHECKS TO INCREASE.

CHAP. 111.

the more vigorous plants gradually kill the less vigorous,
though fully grown, plants : thus out of twenty species
growing on a little plot of turf (three feet by four) ninespecies perished from the other species being allowed to
grow up freely.
The amount of food for each species of course gives
the extreme limit to which each can increase ; but veryfrequently it is not the obtaining food, but the serving
as prey to other animals, which determines the average
numbers of a species. Thus, there seems to be little
doubt that the stock of partridges, gronse, and hares on
any large estate depends chiefly on the destruction ofvermin. If not one head of game were shot durinv the?
next twenty years in England, and, at the same time,
if no vermin were destroyed, there would, in all probability, be less game than at present, although hundredsof thousands of game animals are now annually killed.
On the other hand, in some cases, as with the elephantand rhinoceros, none are destroyed by beasts of prey:even the tiger in India most rarely dares to attack ayoung elephant protected by its dam.
Climate plays an important part in determining the
average numbers of a species, and periodical seasons of
extreme cold or drought, I believe to be the mosteffective of all checlrs. I estimated that the winter of
1854-55 destroyed four-fifths of the birds in my own
grounds ; and this is a tremendous destruction, when weremember that ten per cent. is an extraordinarily severemortality from epidemics with man. The action of climate seems at first sight to be quite independent of the
struggle for existence ; but in so far as climate chiefly
acts in reducing food, it brings on the most serere
struggle between the individuals, whether of the sameor of distinct species, which subsist on the same kindof food. Even when climate, for instance extreme

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67

destruction ever so little, and the number of the species
will almost instantaneously increase to any amount.
The face of Nature may be compared to a yielding
surface, with ten thousand sharp wedges packed close
together and driven inwards by incessant blows, sometimes one wedge being struck, and then another withgreater force.
What checks the natural tendency of each species toincrease in number is most obscure. Look at the mostvigorous species ; by as mach as it swarms in numbers, by
so much will its tendency to increase be still further increased. We know not exactly what the checks are ineven one single instance. Nor will this surprise any one
who reflects how ignorant we are on this head, even in
regard to mankind, so incomparably better known thanany other animal. This subject has been ably treated byseveral authors, and I shall, in my future work, discuss
some of the checks at considerable length, more especiallyin regard to the feral animals of South America. HereI will make only a few remarks, just to recall to thereader’s mind some of the chief points. Eggs or very
young animals seem generally to suffer most, but thisis not invariably the case. With plants there is a vast
destruction of seeds, but, from some observations which
I have made, I believe that it is the seedlings whichsuffer most from germinating in ground already thickly
stocked with other plants. Seedlings, also, are destroyedin vast numbers by various enemies ; for instance, on apiece of ground three feet long and two wide, dug andcleared, and where there could be no choking from otherplants, I marked all the seedlings of our native weeds as
they came up, and out of the 357 no less than 295 weredestroyed, chiefly by slugs and insect,s. If turf which
has long been mown, and the case would be the samewith turf closely browsed by quadrupeds, be let to grow,

6s

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CHAP. 111.

the more vigorous plants gradually kill the less vigorous,
though fully grown, plants : thus out of twenty species
growing on a little plot of turf (three feet by four) ninespecies perished from the other species being allowed to
grow up freely.
The amount of food for each species of coiirse gives
the extreme limit to which each can increase ; but veryfrequently it is not the obtaining food, but the serving
as prey to other animals, which determines the average
numbers of a species. Thus, there seems to be little
doubt that the stock of partridges, grouse, and hares on
any large estate depends chiefly on the destruction ofvermin. If not one head of game were shot durinv thea.
next twenty years in England, and, at the same time,
if no vermin were destroyed, there would, in all probability, be less game than at present, although hundredsof thousands of game animals are now annually killed.
On the other hand, in some cases, as with the elephantand rhinoceros, none are destroyed by beasts of prey:even the tiger in India most rarely dares to attack ayoung elephant protected by its dam.
Climate plays an important part in determining the
average numbers of a species, and periodical seasons of
extreme cold or drought, I believe to be the mosteffective of all checks. I estimated that the winter of
1854-55 destroyed four-fifths of the birds in my owii
grounds ; and this is a tremendous destruction, when weremember that ten per cent. is an extraordinarily severemortality from epidemics with man. The action of climate seems at first sight to be quite independent of the
struggle for existence ; but in so far as climate chiefly
acts in reducing food, it brings on the most serere
struggle between the individuals, whether of the sameor of distinct species, which subsist on the same kindof food. Even when climate, for instance extreme

CHAP. 111.

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69

cold, acts directly, it will be the least vigorous, or thosewhich have got least food through the advancing winter,
which will suffer most. When we travel from south tonorth, or from a damp region to a dry, we invariably
see some species gradually getting rarer and rarer, andfinally disappearing ; and the change of climate being
conspicuous, we are tempted to attribute the whole
effect to its direct action. But this is a very false view :we forget that each species, even where it most abounds,
is constantly suffering enormous destruction at someperiod of its life, from enemies or from competitors forthe same place and food ; and if these enemies or competitors be in the least degree favoured by any slightchange of climate, they will increase in numbers, and,as each area is already fully stocked with inhabitants,
the other species will decrease. When we travel southward and see a species decreasing in numbers, we may
feel sure that the cause lies quite as much in other species being favoured, as in this one being hurt. So it iswhen we travel northward, but in a somewhat lesser
deg-ree, for the number of species of all kinds, and therefore of competitors, decreases northwards ; hence ingoing northward, or in ascending a mountain, we far
oftener meet with stunted forms, due to the directly
injurious action of climate, than we do in proceeding
southwards or in descending a mountain. When wereach the Arctic regions, or snow-capped summits, orabsolute deserts, the struggle for life is almost exclusively with the elements.
That climate acts in main part indirectly by favouringother species, we may clearly see in the prodigious
number of plants in our gardens which can perfectly
well endure our climate, but which never become naturalised, for they cannot compete with our native plants,
nor resist destruction by our native animals.

70

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CHAP. 111.

When a species, owing to highly favourable circumstances, increases inordinately in numbers in a small
tract, epidemics-at least, this seems generally to occur
with our game animals-often ensue : and here we havea limiting check independent of the struggle for life.
But even some of these so-called epidemics appear tobe due to parasitic worms, which have from some cause,
possibly in part through facility of diffusion amongst
the crowded animals, been disproportionably favoured :and here comes in a sort of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock ofindividuals of the same species, relatively to the numbers of its enemies, is absolutely necessary for its preservation. Thus we can easily raise plenty of corn andrape-seed, &c., in our fields, because the seeds are in
great excess compared with the number of birds which
feed on them ; nor can the birds, though having a superabundance of food at this one season, increase in number
proportionally to the supply of seed, as their numbers
are checked during winter: but any one who has tried,
knows how troublesome it is to get seed from a few
wheat or other such plants in a garden ; I have in thiscase lost every single seed. This view of the necessityof a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature, suchas that of very rare plants being sometimes extremely
abundant in the few spots where they do occur; andthat of some social plants being social, that is, abounding in individuals, even on the extreme confines of theirrange. For in such cases, we may believe, that a plant
could exist only where the conditions of its life were sofavourable that many could exist together, and thus saveeach other from utter destruction. I should add that thegood effects of frequent intercrossing, and the ill effects

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71

of close interbreeding, probably come into play in someof these cases ; but on this intricate subject I will nothere enlarge.
Many cases are on record showing how complex
and unexpected are the checks and relations between
organic beings, which have to struggle together in thesame country. I will give only a single instance, which,though a simple one, has interested me. In Staffordshire, on the estate of a relation where I had ample
means of investigation, there was a large and extremely
barren heath, which had never been touched by thehand of man ; but several hundred acres of exactly thesame nature had been enclosed twenty-five years previously and planted with Scotch fir. The change in thenative vegetation of the planted part of the heath wasmost remarkable, more than is generally seen in passing
from one quite different soil to another: not only the
proportional numbers of the heath-plants were whollychanged, but twelve species of plants (not counting
grasses and carices) flourished in the plantations, which
could not be found on the heath. The effect on theinsects must have been still greater, for six insectivorous
birds were very common in the plantations, which were
not to be seen on the heath; and the heath was frequented by two or three distinct insectivorous birds.
Here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having
been done, with the exception that the land had been
enclosed, so that cattle could not enter. But how important an element enclosure is, I plainly saw near
Farnham, in Surrey. Here there are extensive heaths,
with a few clumps of old Scotch firs on the distant hilltops : within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in
multitudes, so close together that all cannot live.

72

MUTUAL CHECKS TO INCREASE.

CHAP. 111.

When I ascertained that these young trees had notbeen sown or planted, I was so much surprised at their
numbers that I went to several points of view, whence
I could examine hundreds of acres of the unenclosed
heath, and literally I could not see a single Scotch fir,except the old planted clumps. But on looking closely
between the stems of the heath, I found a multitude of
seedlings and little trees, which had been perpetuallybrowsed down by the cattle. I n one square yard, at apoint some hundred yards distant from one of the oldclumps, I counted thirty-two little trees ; and one of them,judging from the rings of growth, had during twenty-six
years tried to raise its head above the stems of the heath,and had failed. No wonder that, as soon as the land
was enclosed, it became thickly clothed with vigorously
growing young firs. Yet the heath was so extremely
barren and so extensive that no one would ever haveimagined that cattle would have so closely and effectually searched it for food.
Here we see that cattle absolutely determine theexistence of the Scotch fir ; but in several parts of theworld insects determine the existence of cattle. Perhaps
Paraguay offers the most curious instance of this ; forhere neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in aferal state; and Azara and Rengger have shown that
this is caused by the greater number in Paraguay of acertain fly, which lays its eggs in the navels of theseanimals when first born. The increase of these flies,
numerous as they are, must be habitually checked bysome means, probably by birds. Hence, if certain insectivorous birds (whose numbers are probably regulatedby hawks or beasts of prey) were to increase in Paraguay,
the flies would decrease-then cattle and horses would
become feral, and this would certainly greatly alter (asCHAP. 111.

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73

indeed I have observed in parts of South America) thevegetation : this again would largely affect the insects ;and this, as we just have seen in Staffordshire, theinsectivorous birds, and so onwards in ever-increasing
circles of complexity. We began this series by insectivorous birds, and we have ended with them. Not thatin nature the relations can ever be as simple as this.
Battle within battle must ever be recurring with varying
success; and yet in the long-run the forces are so
nicely balanced, that the face of nature remains uniform
for long periods of time, though assuredly the merest
trifle would often give the victory to one organic being
over another. Nevertheless so profound is our ignorance,and so high our presumption, that we marvel when wehear of the extinction of an organic being ; and as we donot see the cause, we invoke cataclysms to desolate theworld, or invent laws on the duration of the forms of life !
I am tempted to give one more instance showing how
plants and animals, most remote in the scale of nature,are bound together by a web of complex relations. I
shall hereafter have occasion to show that the exotic
Lobelia fulgens, in this part of England, is never visitedby insects, and consequently, from its peculiar structure,never can set a seed, Many of our orchidaceous plants
absolutely require the visits of moths to remove theirpollen-masses and thus to fertilise them. I have, also,
reason to believe that humble-bees are indispensable tothe fertilisation of the heartsease (Viola tricolor), forother bees do not visit this flower. From experiments
which I have tried, I have found that the visits of bees,
if not indispensable, are at least highly beneficial to thefertilisation of our clovers ; but humble-bees alone visit
the common red clover (Trifolium pratense), as otherbees cannot reach the nectar. Hence I have very little
doubt, that if the whole genus of humble-bees became

74

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CHAP. 111.

extinct or very rare in England, the heartsease and red
clover would become very rare, or wholly disappear.
The number of humble-bees in any district depends ina great degree on the number of field-mice, which destroy their combs and nests ; and Mr. H. Newman, who
has long attended to the habits of linmble-bees, believes
that ‘‘more than two-thirds of them are thus destroyed
all over England.” Now the number of mice is largely
dependent, as every one knows, on the number of cats ;and Mr. Newman says, “ Near villages and small towns
I have found the nests of humble-bees more nunierous
than elsewhere, which I attribute to the number of catsthat destroy the mice.” Hence it is quite credible thatthe presence of a feline animal in large numbers in a
district might determine, through the intervention firstof mice and then of bees, the frequency of certain
flowers in that district !
I n the case of every species, many different checks,
acting at different periods of life, and during different
seasons or years, probably come into play; some one
check or some few being generally the most potent, but
all concurring in determining the average number oreven the existence of the species. I n some cases it canbe shown that widely-different checks act on the same
species in different districts. When we look at theplants and bushes clothing an entangled bank, we are
tempted to attribute their proportional numbers andkinds to what we call chance. But how false a viewis this! Every one has heard that when an American
forest is cut down, a veiy different vegetation springs
up; but it has been observed that the trees now
growing on the ancient Indian mounds, in the Southern
United States, display the same beautiful diversity andproportion of kinds as in the surrounding virgin forests.
What a struggle between the several kinds of trees

CHAP. 111.

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75

must here have gone on during long centuries, each annually scattering its seeds by the thousand ; what war
between insect and insect-between insects, snails, andother animals with birds and beasts of prey-all striving
to increase, and all feeding on each other or on the trees
or their seeds and seedlings, or on the other plants which
first clothed the ground and thus checked the growth of
the trees ! Throw up a handful of feathers, and all must
fall to the ground according to definite laws ; but how
simple is this problem compared to the action and reaction of the innumerable plants and animals which havedetermined, in the course of centuries, the proportional
numbers and kinds of trees now growing on the old
Indian ruins !
The dependency of one organic being on another, aa
of a parasite on its prey, lies generally between beings
remote in the scale of nature. This is often the casewith those which may strictly be said to struggle witheach other for exist,ence, as in the case of locusts andgrass-feeding quadrupeds. But the struggle almost invariably will be most severe between the individuals of
the same species, for they frequent the same districts,
require the same food, and are exposed to t,he same
dangers. I n the case of varieties of the same species,
the struggle will generally be almost equally severe,
and we sometimes see the contest soon decided: for
instance, if several varieties of wheat be sown together,and the mixed seed be resown, some of the varieties
which best suit the soil or climate, or are naturally themost fertile, will beat the others and so yield moreseed, and will consequently in a few years quite SUPplant the other varieties. To keep up a mixed stockof even such extremely close varieties as the variously
coloured sweet-peas, they must be each year harvested
separately, and the seed then mixed in due propor-

76

STRUGGLE FOR EXISTENCE.

CHAP. 111.

tion, otherwise the weaker kinds will steadily decreasein numbers and disappear. So again with the varietiesof sheep: it has been asserted that certain mountainvarieties will starve out other mountain-varieties, sothat they cannot be kept together. The same result
has followed from keeping together different varietiesof the medicinal leech. It may even be doubted whether the varieties of any one of our domestic plants
or animals have so exactly the same strength, habits,
and constitution, that the original proportions of amixed stock could be kept up for half a dozen generations, if they were allowed to struggle together, likebeings in a state of nature, and if the seed or youngwere not annually sorted.
As species of the same genus have usually, thoug!iby no means invariably, some similarity in habits andconstitution, and always in structure, the struggle will
generally be more severe between species of the same
genus, when they come into competition with each other,than between species of distinct genera. We see this in
the recent extension over parts of the United States of
one species of swallow having caused t,he decrease of another species. The recent increase of the missel-thrushin parts of Scotland has caused the decrease of thesong-thrush. How frequently we hear of one speciesof rat taking the place of another species under themost different climates! I n Russia the small Asiatic
cockroach has everywhere driven before it its great
congener. One species of charlock will supplant another,and so in other cases. We can dimly see why the competition should be most severe between allied forms,
which fill nearly the same place in the economy of nature ;but probably in no one case could we precisely say why
one species has been victorious over another in thegreat battle of life.

CHAP.111.

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77

A corollary of the highest importance may be deduced
from the foregoing remarks, namely, that the structureof every organic being is related, in the most essential
yet often hidden manner, to that of all other organic
beings, with which it comes into competition for food orresidence, or from which it has to escape, or on which it
preys. This is obvious in the structure of the teeth andtalons of the tiger ; and in that of the legs and claws of
the parasite which clings to the hair on the tiger’s body.
But in the beautifully plumed seed of the dandelion,
and in the flattened and fringed legs of the water-beetle,
the relation seems at first confined to the elements of
air and water. Yet the advantage of plumed seeds no
doubt stands in the closest relation to the land being
already tliickly clothed by other plants; so that theseeds may be widely distributed and fall on unoccupied ground. In the water-beetle, the structure of
its legs, so well adapted for diving, allows it tocompete with other aquatic insects, to hunt, for
its own prey, and to escape serving as prey to otheranimals.
The store of nutriment laid up within the seeds ofmany plants seems at first sight to have no sort ofrelation to other plants. But from the strong growthof young plants produced from such seeds (as peas
and beans), when sown in the midst of long grass,
I suspect that the chief use of the nutriment in theseed is to favour the growth of the young seedling,whilst struggling with other plants growing vigorously
all around.
Look at a plant in the midst of its range, why does
it not double or quadruple its numbers? We knowthat it can perfectly well withstand a little more heator cold, dampness or dryness, for elsewhere it ranges

78

STRUGGLE FOR EXISTENCE.

CHAP.111.

into slightly hotter or colder, damper or drier districts.
I n this case we can clearly see that if we wished in
imagination to give the plant the power of increasing
in number, we should have to give it some advantage
over its competitors, or over the animals which preyedon it. On the confines of its geographical range, a changeof constitution with respect to climate would clearlybe an advantage to our plant; but we have reason
to believe that only a few plants or animals range sofar, that they are destroyed by the rigour of the climate
alone. Not until we reach the extreme confines of life,in the arctic regions or on the borders of an utter desert,
will competition cease. The land may be extremely
cold or dry, yet there will be competition between somefew species, or between the individuals of the same
species, for the warmest or dampest spots.
Hence, also, we can see that when a plant or animal
is placed in a new country amongst new competitors,
though the climate may be exactly the same as in its
former home, yet the conditions of its life will generally
be changed in an essential manner. If we wished to increase its average numbers in its new home, we shouldhave to modify it in a different way to what we shouldhave done in its native country ; for we should have togive it some advantage over a different set of competitors or enemies.
It is good thus to try in our imagination to give any
form some advantage over another. Probably in nosingle instance should we know what to do, so as tosucceed. It will convince us of our ignorance on themutual relations of all organic beings ; a conviction asnecessary, as it seems to be difficult to acquire. All
that we can do, is to keep steadily in mind that eachorganic being is striving to increase at a geometricalCHAP. 111.

STRUGGLE FOR EXISTENCE.

79

ratio ; that each at some period of its life, during someseason of the year, during each generation or a t intervals, has to struggle for life, and to suffer great destruction. When we reflect on this struggle, we may
console ourselves with the full belief, that the war of
nature is not incessant, that no fear is felt, that deathis generally prompt, and that the vigorous, the healthy,
and the happy survive and multiply.

80

NATUEAL SELECTION.

CHAP. Iv.

C H A P T E R IT.
NATUEAL
SELECTION.
Katnral Selection-its power compared with man's selection-i ts
power on characters of trifling importance-its power a t all ages
and on both sexes-Sexual Selection-011 the generality of intcrcrosses between indiriduals of the samc species--Circumstaiices
favonrahle and unfavonrable to Natural Selection, namely,intercrossing, isolation, nnmher of individuals-Slom actionExtinction caused by Katiiral Sclection-Divergence of Character, related to the diversity of inhahitants of any small area,
and to natnrnlisation-Actionof Natural Selection, throughDivergence of Clraracter and Extinction, on the descendants froma common parent-Explains the Grouping of all 01 garlic bcings.

How will the struggle for existence, discussed too briefly
in the last chapter, act in reward to variation? Can'i
the principle of selection, wliich we have seen is sopotent in the hands of man, apply in nature? I think
we shall see that it can act most effectually. Let it beborne in mind in what an endless number of strange
peculiarities our domestic productions, and, in a lesser
degree, those under nature, vary ; and how strong thehereditary tendency is. TTnder domestication, it may betruly said that the whole organisation becomes in somedegree plastic. Let it be borne in mind how infinitely
complex and close-fitting are the mutual relations of allorgBiiic beings to each other and to their physical conditions of life. Can it, then, be thought improbable,
seeing that variations useful to man have undoubtedly
occurred, that other variations useful in some way toeach being in the great and complex battle of life,
should sometimes occur in the course of thousands ofgenerations? If such do occur, can we doubt (remem-

CHAP. IV.

NATURAL SELECTION.

81

bering that many more individuals are born than canpossibly survive) that individuals having any advantage,
however slight, over others, would have the best
chance of surviving and of procreating their kind?
On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly
destroyed. This preservation of favourable variations
and the rejection of injurious variations, I call Natural
Selection. Variations neither useful nor injurious would
not be affected by natural selection, and would be left
a fluctuating element, as perhaps we see in the species
called polymorphic.
W e shall best understand the probable course ofnatural selection by taking the case of a country undergoing some physical change, for instance, of climate.
The proportional numbers of its inhabitants would
almost immediately undergo a change, and some species
might become extinct. We may conclude, from what
we have seen of the intimate and complex manner inwhich the inhabitants of each country are bound together, that any change in the numerical proportions ofsome of the inhabitants, independently of the changeof climate itself, would most seriously affect many of
the others. If the country were open on its borders,
new forms would certainly immigrate, and this also
would seriously disturb the relations of some of theformer inhabitants. Let it be remembered how powerful
the influence of a single introduced tree or mammal has
been shown to be. But in the case of an island, or ofa country partly surrounded by barriers, into which newand better adapted forms could not freely enter, weshould then have places in the economy of nature which
would assuredly be better filled up, if some of the original inhabitants were in some manner modified ; for,had the area been open to immigration, these same82

NATURAL SELECTION.

CHAP. IV.

places would have been seized on by intruders. I n suchcase, every slight modification, which in the course ofages chanced to arise, and which in any way favoured
the individuals of any of the species, by better adapting
them to their altered conditions, would tend to be preserved; and natural selection would thus have free
scope for the work of improvement.
We have reason to believe, as stated in the firstchapter, that a change in the conditions of life, byspecially acting on the reproductive system, causes orincreases variability; and in the foregoing case theconditions of life are supposed to have undergone achange, and this would manifestly be favourable tonatural selection, by giving a better chance of profitablevariations occurring ; and unless profitable variations do
occur, natural selection can do nothing. Not that, as I
believe, any extreme amount of variability is necessaiy ;as man can certainly produce great results by adding
up in any given direction mere individual differences,
so could Nature, but far more easily, from having incomparably longer time at her disposal. Nor do I believethat any great physical change, as of climate, or any
unusual degree of isolation to check immigration, isacttially necessary to produce new and unoccupied
places for natural selection to fill up by modifying
and improving some of the varying inhabitants. For
as all the inhabitants of each country are struggling
together with nicely balanced forces, extremely slightinodifications in the structure or habits of one inhabitant would often give it an advantage over others;
and still further modifications of the same kind would
often still further increase the advantage. No country
can be named in which all the native inhabitants arenow so perfectly adapted to each other and to thephysical conditions under which they live, that none of

CHAP. IV.

NATURAL SELECTION.

83

them could anyhow be improved; for in all countries,
the natives have been so far conquered by naturalisd
productions, that they have allowed foreigners to takefirm possession of the land. And as foreigners havethus everywhere beaten some of the natives, we may
safely conclude that the natives might have been modified with advantage, so as to have better resisted such
intruders.
As man can produce and certainly has produced agreat result by his methodical and unconscious meansof selection, what may not nature effect ? Man can act
only on external and visible characters: nature cares
nothing for appearances, except in so far as they may
be useful to any being. She can act on every internalorgan, on every shade of constitutional difference, on thewhole machinery of life. Man selects only for his own
good ; Nature only for that of the being which she tends.
Every selected character is fully exercised by her ; andthe being is placed under well-suited conditions of life.
Man keeps the natives of many climates in the same
country ; he seldom exercises each selected characterin some peculiar and fitting manner; he feeds a long
and a short beaked pigeon on the same food; hedoes not exercise a long-backed or long-legged quadruped in any peculiar manner; he exposes sheepwith long and short wool to the same climate. He
does not allow the most vigorous males to struggle
for the females. He does not rigidly destroy all inferior animals, but protects during each varying season,
as far as lies in his power, all his productions. H eoften begins his selection by some half-monstrous form ;or at least by some modification prominent enough tocatch his eye, or to be plainly useful to him. Under
nature, the slightest difference of structure or constitution map well turn the nicely-balanced scale in the

84

NATURAL SELECTION.

CHAP. IT’,

struggle for life, and so be preserved. Wov fleeting are
the wishes and efforts of man ! how short his time ! andconsequently how poor will his products be, comparedwith those accumulated by nature during whole geological periods. Can we wonder, then, that nature’s productions should be far ‘‘ truer ” in character than man’s
productions ; that they should be infinitely better
adapted to the most complex conditions of life, andshould plainly bear the stamp of far higher workmanship ?
It may be said that natural selection is daily andhourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad,preserving and adding up all that is good ; silently andinsensibly working, whenever and wherever opportunity offers, at the improvement of each organic being
in relation to its organic and inorganic conditions oflife. W e see nothing of these slow changes in progress,
until the hand of time has marked the long lapse ofages, and then so imperfect is our view into long pastgeological ages, that we only see that the forms of life
are now different from what they formerly were.
Although natural selection can act only through andfor the good of each being, yet characters and structures,
which w e are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eatinginsects green, and bark-feeders mottled-grey ; thealpine ptarmigan wliite in winter, the red-grouse thecolour of heather, and the black-grouse that of peaty
earth, we must believe that these tints are of service tothese birds and insects in preserving them from danger.
Grouse, if not destroyed a t some period of their lives,would increase in countless numbers j they are known
to suffer largely from birds of prey; and hawks are
guided by eyesight to their prey,-so much so, that on

CHAP.IV.

NATURAL SELECTION.

s5

parts of the Continent persons are warned not to keep
white pigeons, as being the most liable to destruction.
Hence I can see no reason to doubt that natural selection might be most effective in giving the proper colour
to each kind of grouse, and in keeping that colour,
when once acquired, true and constant. iSor ought weto think that the occasional destruction of an animal ofany particular colour would produce little effect: weshould remember how essential it is in a flock of white
sheep to destroy every lamb with the faintest trace of
black. I n plants the down on the fruit and the colourof the flesh are considered by botanists as characters ofthe most trifling importance: yet we hear from an
excellent horticulturist, Downing, that in the UnitedStates smooth-skinned fruits suffer far more from abeetle, a curculio, than those with down ; that purple
plums suffer far more from a certain disease than yellow
plums ; whereas another disease attacks yellom-fleshedpeaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences makea great difference in cultivating the several mrieties,
assuredly, in a state of nature, where the trees would
have to struggle with other trees and with a host ofenemies, such differences would effectually settle which
variety, whether a smoot8hor downy, a yellow or purple
fleshed fruit, should succeed.
I n looking a t many small points of difference between
species, which, as far as our ignorance permits us tojudge, seem to be quite unimportant, we must not forgetthat climate, food, &c., probably produce some slightand direct effect. It is, however, far more necessary
to bear in mind that there are many unknown laws ofcorrelation of growth, which, when one part of theorganisation is modified through variation, and themodifications are accumulated by natural selection for

86

NATURAL SELECTION.

CHAP. Iv.

the good of the being, will cause other modifications,
often of the most unexpected nature.
As we see that those variations which under domestication appear at any particular period of life, tend toreappear in the offspring at the same period;-for instance, in the seeds of the many varieties of our culinary
and agricultural plants; in the caterpillar and cocoon
stages of the varieties of the silkworm ; in the eggs ofpoultry, and in the colour of the down of their chickens ;in the horns of our sheep and cattle when nearly adult ;so in a state of nature, natural selection will be enabled
to act on and modify organic beings at any age, by theaccumulation of profitable variations at that age, and by
their inheritance at a corresponding age. If it profit aplant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in thisbeing effected through natural selection, than in the
cotton-planter increasing and improving by selection
the down in the pods on his cotton-trees. Natural
selection may modify and adapt the larva of an insect
to a score of contingencies, wholly different from thosewhich concern the mature insect. These modifications
will no doubt affect, through the laws of correlation, thestructure of the adult ; and probably in the case of those
insects which live only for a few hours, and which neverfeed, a large part of their structure is merely the correlated result of successive changes in the structure of
their laixe. So, conversely, modifications in the adultwill probably often affect the structure of the larva ; but
in all cases natural selection will ensure that modiiications consequent on other modifications at a different
period of life, shall not be in the least degree injurious :for if they became so, they would cause the extinctionof the species.
Natural selection mill modify the structure of theCHAP. Iv.

SEXUAL SELECTION.

87

young in relation to the parent, and of the parent inrelation to the young. I n social animals it will adapt
the structure of each individual for the benefit of thecommunity ;if each in consequence profits by the selectedchange. What natural selection cannot do, is to modify
the structure of one species, without giving it any
advantage, for the good of another species ; and thoughstatements to this effect may be found in works ofnatural history, I cannot find one case which will bearinvestigation. A structure used only once in an animal's
whole life, if of high importance to it, might be modified to any extent by natural selection ; for instance,
the great jaws possessed by certain insects, and used
exclusively for opening the cocoon-or the hard tip tothe beak of nestling birds, used for breaking the egg.
It has been asserted, that of the best short-beaked
tumbler-pigeons more perish in the egg than are able toget out of it ; so that fanciers assist in the act of hatching. Now, if nature had to make the beak of a fullgrown pigeon very short for the bird's own advantage, theprocess of modification would be very slow, and there
would be simultaneously the most rigorous selection of
the young birds within the egg, which had the most powerful and hardest beaks, for all with weak beaks would
inevitably perish: or, more delicate and more easily
broken shells might be selected, the thickness of theshell being known to vary like every other structure.
Sexual Selection.-Inasmuch
as peculiarities often
appear under domestication in one sex and become
hereditarily attached to that sex, the same fact probably occurs under nature, and if so, natural selection
will be able to modify one sex in its functional relations
to the other sex, or in relation to wholly different
habits of life in the two sexes, as is sometimes the case

88

SEXUAL SELECTION.

CHAP. 1V.

with insects. And this leads me to say a few words on
what I call Sexual Selection. This depends, not on a,
struggle for existence, but on a struggle between theinales for possession of the females ; the result is notdeath to the unsuccessful competitor, but few or nooffspring. Sexual selection is, therefore, less rigorous
than natural selection. Generally, the most vigorous
males, those which are best fitted for their places innature, will leave most progeny. But in many cases,
victory will depend not on general vigour, but on having
special weapons, confined to the male sex. A hornlessstag or spiirless cock would have a poor chance ofleaving offspring. Sexual selection by always allowing
the victor to breed might surely give indomitablecourage, length to the spur, and strength to the wing
to strike in the spurred leg, as well as the brutal cockfighter, who knows well that he can improve his breedby careful selection of the best cocks. How low inthe scale of nature this law of battle descends, I know
not ; male alligators have been described as fighting,bellowing, and whirling round, like Indians in a wardance, for the possession of the females ; rnale salmons
have been seen fighting all day long ; male stag-beetles
often bear wounds from the huge mandibles of othermales. Tlie war is, perhaps, severest between the malesof polygamous animals, and these seem oftenest provided with special weapons. The males of carnivorous
animals are already well armed ; tliougli to thein and toothers, special means of defence may be given throughmeans of sexual selection, as the mane to the lion, theshoulder-pad to the boar, and the hooked jaw to themale salmon; for the shield may be as important for
victory, as the sword or spear.
Amongst birds, the contest is often of a more peacefulcharacter. All those mho have attended to the subject,

CHAP. Iv.

SEXUAL SELECTION.

89

believe that there is the severest rivalry between themales of many species to attract by singing the females.
The rock-thrush of Guiana, birds of Paradise, and someothers, congregate ; and successive males display theirgorgeous plumage and perform strange antics before thefemales, which standing by as spectators, at last choose
the most attractive partner. Those who have closely
attended to birds in confinement well know that theyoften take individual preferences and dislikes : thus
Sir R. Heron has described how one pied peacock waseminently attractive to all his hen birds. It may
appear childish to at,tribute any effect to such apparently weak means : I cannot here enter on the details
necessary to support this view ; but if man can in a shorttime give elegant carriage and beauty to his bantams,
according to his standard of beauty, I can see no goodreason to doubt that female birds, by selecting, during
thousands of generations, the most melodious or beautiful
males, according to their standard of beauty, might produce a marked effect. I strongly suspect that somewell-known laws with respect to the plumage of maleand female birds, in comparison with the plumage of theyoung, can be explained on the view of plumage having
been chiefly modified by sexual selection, acting when
the birds have come to the breeding age or during thebreeding season ; the modifications thus produced being
inherited at corresponding ages or seasons, either by
the males alone, or by the males and females; but I
have not space here to enter on this subject.
Thus it is, as I believe, that when the males andfemales of any animal have the same general habitsof life, but differ in structure, colour, or ornament,such differences have been mainly caused by sexual
selection; that is, individual males have had, in successive generations, some slight advantage over other

90

NATURAL SELECTION.

CHAP. 11’.

males, in their weapons, means of defence, or charms ;and have transmitted these advantages to their maleoffspring. Yet, I would not wish to attribute all suchsexual differences to this agency: for we see peculiarities arising and becoming attached to the male sex
in our domestic animals (as the wattle in male carriers,
horn-like protuberances in the cocks of certain fowls,&.), which we cannot believe to be either useful to themales in battle, or attractive to the females. We see
analogous cases under nature, for instance, the tuft ofhair on the breast of the turkey-cock, which can hardly
be either useful or ornamental to this bird;-indeed,
liad the tuft appeared under domestication, it would
have been called a monstrosity.
IZZustrations of the action of Natural 8dection.-In
order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two
imaginary illustrations. Let us take the case of a wolf,
which preys on various animals, securing some by craft,some by strength, and some by fleetness; and let us
suppose that the fleetest prey, a deer for instance, hadfrom any change in the country increased in numbers,
or that other prey had decreased in numbers, duringthat season of the year when the wolf is hardest pressed
for food. I can under such circumstances see no reason
to doubt that the swiftest and slimmest wolves would
have the best chance of surviving, and so be preservedor selected,-provided always that they retained strength
to master their prey at this or at some other period of
the year, when they might be compelled to prey onother animals. I can see no more reason to doubt this,than that man can improve the fleetness of his greyhounds by careful and methodical selection, or by that
unconscious selection which results from each man trying

CHAP. IV.

NATURAL SELECTION.

91

to keep the best dogs without m y thought of modifying
the breed
Even without any change in the proportional numbersof the animals on which our wolf preyed, a cub might
be born with an innate tendency to pursue certain kindsof prey. Nor can this be thought very improbable ; for
we often observe great differences in the natural tendencies of our domestic animals ; one cat, for instance,
taking to catch rats, another mice ; one cat, according to
Mr. St. John, bringing home winged game, another haresor rabbits, and another hunting on marshy ground andalmost nightly catching woodcocks or snipes. The tendency to catch rats rather than mice is known to be inherited. Now, if any slight innate change of habit or ofstructure benefited an individual wolf, it would havethe best chance of surviving and of leaving offspring.
Some of its young would probably inherit the same
habits or structure, and by the repetition of this process,a new variety might be formed which would either supplant or coexist with the parent-form of wolf‘. Or, again,
the wolves inhabiting a mountainous district, and thosefrequenting the lowlands, would naturally be forced tohunt different prey ; and from the continued preservation of the individuals best fitted for the two sites, twovarieties might slowly be formed. These varieties wouldcross and blend where they met; but to this subject ofintercrossing we shall soon have to return. I may add,
that, according to Mr. Pierce, there are two varieties of
the wolf inhabiting the Catskill Mountains in the UnitedStates, one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter legs,
which more frequently attacks the shepherd’s flocks.
Let us now take a more complex case. Certain plants
excrete a sweet juice, apparently for the sake of eliminating something injurious from their sap: this is

92

N4TURAL SELECTION.

CHAP. IV.

effected by glands at the base of the stipules in some
Leguminosae, and at the back of the leaf of the common
laurel. This juice, though small in quantity, is greedilysought by insects. Let us now suppose a little sweet
juice or nectar to be excreted by the inner bases of thepetals of a flower. I n this case insects in seeking thenectar would get dusted with pollen, and would certainly
often transport the pollen from one flower to the stigma
of another flower. The flowers of two distinct illdividualsof the same species would thus get crossed ; and the actof crossing, we have good reason to believe (as will
hereafter be more fully alluded to), would produce very
vigorous seedlings, which consequently would have thebest chance of flourishing and surviving. Some of theseseedlings would probably inherit the nectar-excreting
power. Those individual flowers which had the largest
glands or nectaries, and which excreted most nectar,
would be oftenest visited by insects, and would beoftenest crossed; and so in the long-run would gain
the upper hand. Those flowers, also, which had theirstamens and pistils placed, in relation to the size andhabits of the particular insects which visited them, so as
to favour’ in any degree the transportal of their pollen
from flower to flower, would likewise be favoured or aelected. We might have taken the case of insects
visiting flowers for the sake of collecting pollen insteadof nectar; and as pollen is formed for the sole objectof fertilisation, its destruction appears a simple loss tothe plant; yet if a little pollen were carried, at firstoccasionally and then habitnally, by the pollen-devouring insects from flower to flower, and a cross thus
eifected, although nine-tenths of the pollen were destroyed, it might still be a great gain to the plant ; andthose individuals which produced more and more pollen,
and had larger and larger anthers, would be selected.

CHAP. IV.

NATURAL SELECTION.

93

When our plant, by this process of the continued
preservation or natural selection of more and more
attractive flowers, had been rendered highly attractive
to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower ; and thatthey can most effectually do this, I could easily showby many striking instances. I will give only one-not
as a very striking case, but as likellise illustrating one
step in the separation of the sexes of plants, presently
to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather asmall quantity of pollen, and a rudimentary pistil;other holly-trees bear only female flowers ; these have afull-sized pistil, and four stamens with shrivelled anthers,
in which not a grain of pollen can be detected. Having
found a female tree exactly sixty yards from a maletree, I put the stigmas of twenty flowers, taken fromdifferent branches, under the microscope, and on all,without exception, there were pollen-grains, and onsome a profusion of pollen. As the wind had set forseveral days from the female to the male tree, thepollen could not thus have been carried. The weather
had been cold and boisterous, and therefore not favourable to bees, nevertheless every female flower which
I examined had been effectually fertilised by the bees,
accidentally dusted with pollen, having flown from tree
to tree in search of nectar. But to return to ourimaginary case: as soon as the plant had been rendered so highly attractive to insects that pollen nas
regularly carried from flower to flower, anotherprocess might commence. No naturalist doubts theadvantage of what has been called the ‘‘physiologicaldivision of labour ;” hence we may believe that it would
be advantageous to a plant to produce stamens alone in
one flower or on one whole plant, and pistils alone in94

NATURAL SELECTION.

CHAP.

Iv.

another flower or on another plant. I n plants underculture and placed under new conditions of life, sometimes the male organs and sometimes the female organs
become more or less impotent ; now if we suppose this tooccur in ever so slight a degree under nature, then aspollen is already carried regularly from flower to flower,
and as a more complete separation of the sexes of ourplant would be admntageous on the principle of thedivision of labour, individuals with this tendency more
and more increased, would be continually favoured or
selected, until at last a complete separation of the sexes
would be effected.
Let us now turn to the nectar-feeding insects in our
imaginary case: we may suppose the plant of which
we have been slowly increasing the nectar by continued
selection, to be a common plant ; and that certain insects depended in main part on its nectar for food. I
could give many facts, showing how anxious bees areto save time; for instance, their habit of cutting
holes and sucking the nectar at the bases of certain
flowers, which they can, with a very little more trouble,
enter by the mouth. Bearing such facts in mind, I can
see no reason to doubt that an accidental deviation in
the size and form of the body, or in the curvature andlength of the proboscis, &c., far too slight to be appreciated by us, might profit a bee or other insect, so thatan individual so characterised would be able to obtainits food more quickly, and so have a better chance ofliving and leaving descendants. Its descendants would
probably inherit a tendency to a similar slight deviationof structure. The tubes of the corollas of the common
red and incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty glance appear to differ inlength ; yet the hive-bee can easily suck the nectar outof t,he incarnate clover, but not out of the common red

CHAP.1V.

NATURAL SELECTION.

95

clover, which is visited by humble-bees alone ; so thatwhole fields of the red clover offer in vain an abundant
supply of precious nectar to the hive-bee. Thus it
might be a great advantage to the hive-bee to have aslightly longer or differently constructed proboscis. On
the other hand, I have found by experiment that thefertility of clover greatly depends on bees visiting andmoving parts of the corolla, so as to push the pollen
on to the stigmatic surface. Hence, again, if humblebees were to become rare in any country, it might
be a great advantage to the red clover to have ashorter or more deeply divided tube to its corolla, sothat the hive-bee could visit its flowers. Thus I canunderstand how a flower and a bee might slowly become, either simultaneously or one after the other,modified and adapted in the most perfect manner toeach other, by the continued preservation of individuals
presenting mutual and slightly favourable deviations ofstructure.
I am well aware that this doctrine of natural selection,
exemplified in the above imaginary instances, is open to
the same objections which were at first urged against
Sir Charles Lyell’s noble views on “ the modern changesof the earth, as illustrative of geology;” but we now
very seldom hear the action, for instance, of the coastwaves, called a trifling and insignificant cause, whenapplied to the excavation of gigantic valleys or to theformation of the longest lines of inland cliffs. Natural
selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, eachprofitable to the preserved being; and as modern
geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so
will natural selection, if it be a true principle, banish
the belief of the continued creation of new organic

96

ON THE ADVANTAGE

CHAP. IV.

beings, or of any great and sudden modification in theirstructure.
On the Intercrossing of Individuals.-I
must here
introduce a short digression. I n the case of animals
and plants with separated sexes, it is of course obviousthat two individuals must always unite for each birth ;but in the case of hermaphrodites this is far from o bvious. Nevertheless I am strongly inclined to believethat with all hermaphrodites two individuals, either
occasionally or habitually, concur for the reproduction of
their kind. This view, I may add, was first suggested
by Andrew Knight. We shall presently see its importance ; but I mast here treat the subject with extreme
brevity, though I have the materials prepared for an
ample discussion. All vertebra.te animals, all insects,
and some other large groups of animals, pair for eachbirth. Modern research has much diminished thenumber of supposed hermaphrodites, and of real hermaphrodites a large number pair ; that is, two individuals regularly units for reproduction, which is all thatconcerns us. But still there are many hermaphrodite
animals which certainly do not habitually pair, and avast majority of plants are hermaphrodites. Whatreason, it may be auked, is there for supposing in thesecases that two individuals ever concur in reproduction?
As it is impossible here to enter on details, I must trust
to some general considerations alone.
In the first place, I have collected so large a body offacts, showing, in accordance with the almost universal
belief of breeders, that with animals and plants a crossbetween different varieties, or between individuals of
the same variety but of another strain, gives vigour andfertility to the offspring; and on the other hand, thatclose interbreeding diminishes vigour and fertility ; that

CHAP.IV.

OF INTERCROSSING.

97

these facts alone incline me to believe that it is a
general law of nature (utterly ignorant though we beof the meaning of the law) that no organic being selffertilises itself for an eternity of generations ; but that across with another individual is occasionally-perhaps
at very long intervals-indispensable.
On the belief that this is a law of nature, we can, I
think, understand several large classes of facts, such as
the following, which on any other view are inexplicable.
Every hybridizer knows how unfavourable exposure to
wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! but if an occasional cross be
indispensable, the fullest freedom for the entrance ofpollen from another individual will explain this stateof exposure, more especially as the plant’s own anthers
and pistil generally stand so close together that selffertilisation seems almost inevitable. Many flowers, on
the other hand, have their organs of fructification closely
enclosed, as in the great papilionaceous or pea-family ;but in several, perhaps in all, such flowers, there is avery curious adaptation between the structure of theflower and the manner in which bees suck the nectar ;for, in doing this, they either push the flower’s own pollen
on the stigma, or bring pollen from another flower. So
necessary are the visits of bees to papilionaceous flowers,that I have found, by experiments published elsewhere,that their fertility is greatly diminished if these visits
be prevented. Now, it is scarcely possible that bees
should fly from flower to flower, and not carry pollen
from one to the other, to the great good, as I believe,of the plant. Bees will act like a camel-hair pencil,
and it is quite sufficient just to touch the anthers of
one flower and then the stigma of another with thesame brush to ensure fertilisation : but it must not be

98

OX THE ADVANTAGE

CIIAP. 1V.

supposed that bees would thus produce a multitude of
hybrids between distinct species ; for if you bring on thesame brush a plant’s own pollen and pollen from anotherspecies, the former will have such a prepotent effect,that it will invariably and completely destroy, as has beenshown by Gartner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards
the pistil, or slowly move one after the other towards it,
the contrivance seems adapted solely to ensure selffertilisation ; and no doubt it is useful for this end : but,the agency of insects is often required to cause thestamens to spring forward, as Kolreuter has shown tobe the case with the barberry; and curiously in thisvery genus, which seems to have a special contrivancefor self-fertilisation, it is well known that if very closelyallied forms or varieties are planted near each other, it
is hardly possible to raise pure seedlings, so largely dothey naturally cross. I n many other cases, far fromthere being any aids for self-fertilisation, there are
special contrivances, as I could show from the writingsof C. C. Sprengel and from my own observations, which
effectually prevent the stigma receiving pollen from its
own flower: for instance, in Lobelia fulgens, there is areally beautiful and elaborate contrivance by which
every one of the infinitely numerous pollen-granules
are swept out of the conjoined anthers of each flower,
before the stigma of that individual flower is ready toreceixe them ; and as this flower is never visited, at least
in my garden, by insects, it never sets a seed, though byplacing pollen from one flower on the stigma of another,
I raised plenty of seedlings ; and whilst another speciesof Lobelia growing close by, which is visited by bees,
seeds freely. I n very many other cases, though there
be no special mechanical contrivance to prevent thestigma of a flower receiving its own pollen, yet, asCHAP. 1V.

OF INTERCROSSING.

99

C. C. Sprengel has shown, and as I can confirm, either theanthers burst before the stigma is ready for fertilisation,or the stigma is ready before the pollen of that flower
is ready, so that these plants have in fact separated
sexes, and must habitually be crossed. How strange
are these facts! How strange that the pollen and stigmatic surface of the same flower, though placed so close
together, aa if for the very purpose of self-fertilisation,
should in so many cases be mutually useless to eachother! How simply are these facts explained on theview of an occasional cross with a distinct individual
being advantageous or indispensable !
If several varieties of the cabbage, radish, onion, andof some other plants, be allowed t o seed near each other,a large majority, as I have found, of the seedlings t 11s
raised will turn out mongrels : for instance, I raised 233
seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true
to their kind, and some even of these were not perfectlytrue. Yet the pistil of each cabbage-flower is surrounded not only by its own six stamens, but by thoseof the many other flowers on the same plant. How,
then, comes it that such a vast number of the seedlings
are mongrelized? I suspect that it must arise from thepollen of a distinct variety having a prepotent effectover a flower’s own pollen; and that this is part of
the general law of good being derived from the intercrossing of distinct individuals of the same speciee.
When distinct species are crossed the case is directly
the reverse, for a plant’s own pollen is always prepotent
over foreign pollen ; but to this subject we shall returnin a future chapter.
In the case of a gigantic tree covered with innumerable flowers, it may be objected that pollen could seldom
be carried from tree to tree, and at most only from flower

ib

100

ON THE ADVANTAGE

CHAP.

Iv.

to flower on the same tree, and that flowers on the same
tree can be considered as distinct individuals only in alimited sense. I believe this objection to be valid, butthat nature has largely provided against it by giving totrees a strong tendency to bear flowers with separated
sexes. When the sexes are separated, although themale and female flowers may be produced on the sftnie
tree, we can see that pollen must be regularly carried
from flower to flower ; and this will give a better chanceof pollen being occasionally carried from tree to tree.
That trees belonging to all Orders have their sexes
more often separated than other plants, I find to be thecase in this country; and at my request Dr. Hooker
tabulated the trees of New Zealand, and Dr. Asa Graythose of the United States, and the result was as I anticipated. On the other hand, Dr. Hooker has recently
informed me that he finds that the rule does not holdin Australia ; and I have made these few remarks on
the sexes of trees simply to call attention to the subject.
Turning for a very brief space to animals: on theland there are some hermaphrodites, as land-mollusca
and earth-worms ; but these all pair. As yet I have notfound a single case of a terrestrial animal which fertilises itself. We can understand this remarkable fact,which offers so strong a contrast with terrestrial plants,on the view of an occasional cross being indispensable,by considering the medium in which terrestrial animals
live, and the nature of the fertilising element; for weknow of no means, analogous to the action of insects andof the wind in the case of plants, by which an occasionalcross could be effected with terrestrial animals without
the concurrence of two individuals. Of aquatic animals,
there are many self-fertilising hermaphrodites ; but here
currents in the water offer an obvious means for an occasional cross. And, as in the case of flowers, I have as yet

CHAP. IV.

OF INTERCROSSING.

101

failed, after consultation with one of the highest authorities, namely, Professor Huxley, to discover a single caseof an hermaphrodite animal with the organs of reproduction so perfectly enclosed within the body, that accessfrom without and the occasional influence of a distinct
individual can be shown to be physically impossible.
Cirripedes long appeared to me to present a case ofvery great difficulty under this point of view; but I
have been enabled, by a fortunate chance, elsewhere toprove that two individuals, though both are self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange
anomaly that, in the case of both animals and plants,
species of the same family and even of the same genus,
though agreeing closely with each other in almost theirwhole organisation, y e t a r e not rarely, some of themhermaphrodites, and some of them unisexual. But if, in
fact, all hermaphrodites do occasionally intercross withother individuals, the difference between hemaphrodites
and unisexual species, as far as function is concerned,
becomes very small.
From these several considerations and from the many
special facts which I have collected, but which I am
not here able to give, I am strongly inclined to suspect
that, both in the vegetable and animal kingdoms, anoccasional intercross with a distinct individual is a lawof nature. I am well aware that there are, on this view,
many cases of difficulty, some of which I am trying toinvestigate. Finally then, we may conclude that inmany organic beings, a cross between two individuals isan obvious necessity for each birth ; in many others it
occurs perhaps only at long intervals ; but in none, as I
suspect, can self-fertilisation go on for perpetuity.

Circumstances favourable to Natural Selection.--This

102

CIRCUMST.4XCES FAVOURABLE

CHAP. IV.

is an extremely intricate subject. A large amount ofinheritable and diversified variability is favourable, but
I believe mere individual differences suffice for the work.
A large number of individuals, by giving a better chancefor the appearance within any given period of profitablevariations, will compensate for a lesser amount of variability in each individual, and is, I believe, an extremely important element of success. Though nature
grants vast periods of time for the work of natural
selection, she does not grant an indefinite period; foras all organic beings are striving, it may be said, toseize on each place in the economy of nature, if any one
species does not become modified and improved in acorresponding degree with its competitors, it will soon
be exterminated.
I n man’s methodicd selectim, a breeder selects forsome definite object, and free intercrossing will whollystop his work. But when many men, without intending
to alter the breed, have a nearly common standard ofperfection, and all try to get and breed from the best
animals, much improvement and modification surely but
slowly follow from this unconscious process of selection,
notwithstanding a large amount of crossing with inferior
animals. Thus it will be in nature ; for within a confined area, with some place in its polity not so perfectly
occupied as might be, natural selection will always tend
to preserve all the individuals varying in the right direction, though in different degrees, so as better to fill up
the unoccupied place. But if the area be large, its
several districts will almost certainly present different
conditions of life ; and then if natural selection be modifying and improving a species in the several districts,
there will be intercrossing with the other individualsof the same species on the confines of each. And in
this case the effects of intercrossing can hardly be Colin-

CIIAP. IV.

TO NATURAL SELECTION.

103

terbalanced by natural selection always tending to modify all the individuals in each district in exactly thesame manner to the conditions of each; for in a continuous area, the conditions will generally graduate
away insensibly from one district to another. The intercrossing will most affect those animals which unitc
for each birth, which wander much, and which do notbreed at a very quick rate. Hence in animals of thisnature, for instance in birds, varieties will generally be
confined to separated countries ; and this I believe to be
the case. I n hermaphrodite organisms which cross only
occasionally, and likewise in animals which unite foreach birth, but which wander little and wliich can increase at a very rapid rate, a new and improved variety
might be quiclrly formed on any one spot, and might
there maintain itself in a body, so that whatever intercrossing took place would be chiefly between the individuals of the same new variety. A local variety whenonce thus formed might subsequently slowly spread toother districts. On the above principle, nurserymen
always prefer getting seed from a large body of plantsof the same variety, as the chance of intercrossing withother varieties is thus lessened.
Even in the case of slow-breeding animals, which
unite for each birth, we must not overrate the effects
of intercrosses in retarding natural selection ; for I canbring a considerable catalogue of facts, showing thatwithin the same area, varieties of the same animal canlong remain distinct, from haunting different stations,from breeding at slightly different seasons, or fromvarieties of the same kind preferring to pair together.
Intercrossing plays a very important part in naturein keeping the individuals of the same species, or of thesame variety, true and uniform in character. It will
obviously thus act far more efficiently with those animals

104

CIRCUMSTANCES FAVOURABLE

CHAP. 1V.

which unite for each birth ; but I have already attempted
to show that we have reason to believe that occasional
intercrosses take place with all animals and with allplants. Even if these take place only at long intervals,
I am convinced that the young thus produced will gainso much in vigour and fertility over the offspring fromlong-continued self-fertilisation, that they will have abetter chance of surviving and propagating their kind ;and thus, in the long run, the influence of intercrosses,
even at rare intervals, will be great. If there exist
organic beings which never intercross, uniformity ofcharacter can be retained amongst them, as long as
their conditions of life remain the same, only throughthe principle of inheritance, and through natural selection destroying any which depart from the proper type ;but if their conditions of life change and they undergo
modification, uniformity of character can be given to
their modified offspring, solely by natural selection preserving the same favourable variations.
Isolation, also, is an important element in the processof natural selection. I n a confined or isolated area, if
not very large, the organic and inorganic conditions oflife will generally be in a great degree uniform ; so thatnatural selection will tend to modify all the individuals
of a varying species throughout the area in the samemanner in relation to the same conditions. Intercrosses,
also, with the individuals of the same species, which
otherwise would have inhabited the surrounding and differently circumstanced districts, will be prevented. But
isolation probably acts more efficiently in checking theimmigration of better adapted organisms, after any physical change, such as of climate or elevation of the land,&c.; and thus new places in the natural economy of
the country are left open for the old inhabitants tostruggle for, and become adapted to, through modifica-

CHAP.IV.

TO NATURAL SELECTION.

105

tions in their structure and constitution. Lastly, isolation, by checking immigration and consequently competition, will give time for any new variety to be slowly
improved; and this may sometimes be of importance inthe production of new species. If, however, an isolatedarea be very small, either from being surrounded bybarriers, or from having very peculiar physical conditions, the total number of the individuals supported on
it will necessarily be very small ; and fewness of individuals will greatly retard the production of new species
through natural selection, by decreasing the chance of
the appearance of favourable variations.
If we turn to nature to test the truth of these remarks, and look at any small isolated area, such as an
oceanic island, although the total number of the species
inhabiting it, will be found to be small, as we shall see
in our chapter on geographical distribution; yet ofthese species a very large proportion are endemic,-that
is, have been produced there, and nowhere else. Hencean oceanic island at first sight seems to have beenhighly favourable for the production of new species.
But we may thus greatly deceive ourselves, for to ascertain whether a small isolated area, or a large open area
like a continent, has been most favourable €or the production of new organic forms, we ought to make the comparison within equal times; and this we are incapable of doing.
Although I do not doubt that isolation is of considerable importance in the production of new species, on
the whole I am inclined to believe that largeness ofarea is of more importance, more especially in the
production of species, which will prove capable ofenduring for a long period, and of spreading widely.
Throughout a great and open area, not only will there
be a better chance of favourable variations arising fromthe large number of individuals of the same species

106

CIRCUYSTANCES FAVOURADLE

CHAP. 1V.

there supported, but the conditions of life are infinitely
complex from the large number of already existing
species; and if some of these many species become
modified and improved, others will have to be improvedin a corresponding degree or they will be exterminated.
Each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into competition withmany others. Hence more new places will be formed,
and the competition to fill them will be more severe, on
a large than on a small and isolated area. Noreover,p e a t areas, though now continuous, owing to oscillationsof level, will often have recently existed in a broken condition, so that the good effects of isolation will generally,
to a certain extent, have concurred. Finally, I conclude
that, although small isolated areas probably have beenin some respects highly favourable for the productionof new species, yet that the course of modification will
generally have been more rapid on large areas; andwhat is more important, that the new forms producedon large areas, which already have been victorious overmany competitors, will be those that will spread mostwidely, will give rise t o most new varieties and species,
and will thus play an important part in the changing
history of the organic world.
We can, perhaps, on these views, understand somefacts which will be again alluded to in our chapter on
geographical distribution ; for instance, that the productions of the smaller continent of Australia haveformerly yielded, and apparently are now yielding,
before those of the larger Europaeo-Asiatic area. Thus,also, it is that continental productions have everywhere
become so largely naturalised on islands. On a small
island, the race for life will have been less severe, andthere will have been less modification and less exter-

CHAP.

Iv.

TO NATURAL SELECTION.

107

mination. Hence, perhaps, it comes that the flora ofMadeira, according to Oswald Heer, resembles the extinct
tertiary flora of Europe. All fresh-water basins, taken
together, make a small area compared with that of thesea or of the land ; and, consequently, the competition
between fresh-water productions will have been lesssevere than elsewhere ; new forms will have been more
slowly formed, and old forms more slowly exterminated.
And it is in fresh water that we find seven genera ofGanoid fishes, remnants of a once preponderant order :and in fresh water we find some of the most anomalous
forms now known in the world, as the Ornithorhynchus
and Lepidosiren, which, like fossils, connect to a certain
extent orders now widely separated in the natural scale.
These anomalous forms may almost be called living
fossils; they have endured to the present day, fromhaving inhabited a confined area, and from having thus
been exposed to less severe competition.
To sum up the circumstances favourable and unfavourable-to natural selection, as far as the extreme
intricacy of the subject permits. I conclude, looking
to the future, that for terrestrial productions a large
continental area, which will probably undergo many
oscillations of level, and which consequently will exist
for long periods in a broken condition, will be the mostfavourable for the production of many new forms of life,
likely to endure long and to spread widely. For the area
will first have existed as a continent, and the inhabitants,
a t this period numerous in individuals and kinds, will
have been subjected to very severe competition. When
converted by subsidence into large separate islands,
there will still exist many individuals of the same
species on each island: intercrossing on the confineflof the range of each species will thus be checked: after
physical changes of any kind, immigration will be pre-

10s

NATURAL SELECTION.

CHAP. IV.

vented, so that new places in the polity of each island
will have to be filled up by modifications of the old inhabitants ; and time will be allowed for the varieties ineach to become well modified and perfected. When, by
renewed elevation, the islands shall be re-converted into
a continental area, there will again be severe competition : the most favoured or improved varieties will beenabled to spread: there will be much extinction of
the less improved forms, and the relative proportional
numbers of the various inhabitants of the renewed continent will again be changed; and again there will bea fair field for natural selection to improve still further
the inhabitants, and thus produce new species.
That natural selection will always act with extreme
slowness, I fully admit. Its action depends on there
being places in the polity of nature, which can be better
occupied by some of the inhabitants of the country
undergoing modification of some kind. The existenceof such places will often depend on physical changes,
which are generally very slow, and on the immigrationof better adapted forms having been checked. But theaction of natural selection will probably still oftener depend on some of the inhabitants becoming slowly modified; the mutual relations of many of the other inhabitants being thus disturbed. Nothing can be effected,
unless favourable variations occur, and variation itself isapparently always a very slow process. The process willoften be greatly retarded by free intercrossing. Many
will exclaim that these several causes are amply su%cient wholly to stop the action of natural selection. I do
not believe so. On the other hand, I do believe thatnatural selection mill always act very slowly, often only
at long intervals of time, and generally on only a very
few of the inhabitants of the same region a t the sametime. I further believe, that this very slow, intermit-

CHAP. Iv.

EXTINCTION.

109

tent action of natural selection accords perfectly wellwith what geology tells us of the rate and manner atwhich the inhabitants of this world have changed.
Slow though the process of selection may be, if feeble
man can do much by his powers of artificial selection,
I can see no limit to the amount of change, to thebeauty and infinite complexity of the coadaptations
between all organic beings, one with another and with
their physical conditions of life, which may be effected in
the long course of time by nature's power of selection.
Xztinction.-This subject will be more fully discussed
in our chapter on Geology ; but it must be here alluded
to from being intimately connected with natural selection. Natural selection acts solely through the preservation of variations in some way advantageous, whichconsequently endure. But as from the high geometrical
powers of increase of all organic beings, each area isalready fully stocked with inhabitants, it follows thatas each selected and favoured form increases in number,so will the less favoured forms decrease and becomerare. Rarity, as geology tells us, is the precursor to
extinction. We can, also, see that any form represented by few in ividuals will, during fluctiiationsin the seasons or in the number of its enemies, run
a good chance of utter extinction. But we may go
further than this ; /for as new forms are continuallyand slowly being pioduced, unless we believe that the
number of specific forms goes on perpetually and almost
indefinitely increasing, numbers inevitably must become extinct. That the number of specific forms has
not indefinitely increased, geology shows us plainly ;and indeed we can see reason why they shoula nothave thus increased, for the number of places in thepolity of nature is not indefinitely great,-not that we

d

110

NATURAL SELECTION.

CEAP. IV.

liave any means of knowing that any one region has
as yet got its maximum of species. Probably no regionis as yet fully stocked, for at the Cape of Good Hope,
where more species of plants are crowded together thanin any other quarter of the world, some foreign plants
have become naturalised, without causing, as far as weknow, the extinction of any natives.
Furthermore, the species which are most numerous in individuals will have the best chance of producing within any given period favourable variations.
We have evidence of this, in the facts given in thesecond chapter, showing that it is the common species
which afford the greatest number of recorded varieties,
or incipient species. Hence, rare species will be lessquickly modified or improved within any given period,
and they will consequently be beaten in the race for lifeby the modified descendants of the commoner species.
From these several considerations I think it inevitably follows, that as new species in the course oftime are formed through natural selection, others will
become rarer and rarer, and finally extinct. The forms
which stand in closest competition with those undergoing modification and improvement, will naturally
suffer most. And we have seen in the chapter on the
Struggle for Existence that it is the most closely-allied
forms,-varieties of the same species, and species ofthe same genus or of related genera,-which,fromhaving nearly the same structure, constitution, andhabits, generally come into the severest competition
with each other. Consequently, each new variety or
species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend toexterminate them. We see the same process of extermination amongst our domesticated productions, throughthe selection of improved forms by man. Many curious

CHAP. IV.

DIVERGEYCE OF CHARACTER.

111

instances could be given showing how quickly new breedsof cattle, sheep, and other animals, and varieties offlowers, take the place of older and inferior kinds. I n
Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these‘‘ were swept away by the short-horns” ( I quote thewords of an agricultural writer) “as if by some murderous pestilence.”
Divergence of Character.-The principle, which I havedesignated by this term, is of high importance on mytheory, and explains, as I believe, several important
facts. I n the first place, varieties, even stronglymarked ones, though having somewhat of the characterof species-as is shown by the hopeless doubts in manycases how to rank them-yet certainly differ from eachother far less than do good and distinct species. Kevertheless, according to my view, varieties are species in
the process of formation, or are, as I have called them,incipient species. How, then, does the lesser difference
hetween varieties become augmented into the greater
difference between species ? That this does habitually
happen, we must infer from most of the innumerable
species throughout nature presenting well-marked differences ; whereas varieties, the supposed prototypes
and parents of future well-marked species, present slightand ill-defined differences. Mere chance, as me may
call it, might cause one variety to differ in some character from its parents, and the offspring of this variety
again to differ from its parent in the very same character and in a greater degree; but this alone would
never account for so habitual and large an amount ofdifference as that between varieties of the same species
and species of the same genus.
As has always been my practice, let us seek light 011

112

NATURAL SELECTION.

CHAP. IV.

this head from our domestic productions. We shall here
find something analogous. A fancier is struck by apigeon having a slightly shorter beak ; another fancier
is struck by a pigeon having a rather longer beak ; andon the acknowledged principle that “ fanciers do not
and will not admire a medium standard, but like extremes,” they both go on (as has actually occurredwith tumbler-pigeons) choosing and breeding from birds
with longer and longer beaks, or with shorter andshorter beaks. Again, we may suppose that at an early
period one man preferred swifter horses ; anotherstronger and more bulky horses. The early differences
would be very slight; in the course of time, from thecontinued selection of swifter horses by some breeders,and of stronger ones by others, the differences would
become greater, and Jyould be noted as forming two
sub-breeds ; finally, after the lapse of centuries, the subbreeds would become converted into two well-establisher1
and distinct breeds. As the differences slowly become
greater, the inferior animals with intermediate characters, being neither very swift nor very strong, will
have been neglected, and will have tended to disappear.
Here, then, we see in man’s productions the action ofmhat may be called the principle of divergence, causing
differences, at first barely appreciable, steadily to increase, and the breeds to diverge in character bothfrom each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply
most efficiently, from the simple circumstance that themore diversified the descendants from any one species
become in structure, constitution, and habits, by so much
will they be better enabled to seize on many and widely
diversified places in the polity of nat,ure, and so beenabled to iricrease in numbers.

CIIAP. IV.

DIVERGENCE O F CHARACTER.

113

We can clearly see this in the case of animals withsimple habits. Take the case of a carnivorous quadruped, of which the number that can be supported in any
country has long ago arrived at its full average. If its
natural powers of increase be allowed to act, it can
succeed in increasing (the country not undergoing any
change in its conditions) only by its varying descendants
seizing on places at present occupied by other animals :some of them, for instance, being enabled to feed on
new kinds of prey, either dead or alive ; some inhabiting
new stations, climbing trees, frequenting water, and someperhaps becoming less carnivorous. The more diversified in habits and structure the descendants of our carnivorous animal became, the more places they would beenabled to occupy. What applies to one animal will
apply throughout all time to all animals-that is, if theyvary-for otherwise natural selection can do nothing.
So it will be with plants. It has been experimentally
proved, that if a plot of ground be sown with one speciesof grass, and a similar plot be sown with several distinct
genera of grasses, a greater number of plants and agreater weight of dry herbage can thus be raised. The
same has been found to hold good when first one variety
and then several mixed varieties of wheat have beensown on equal spaces of ground. Hence, if any one
species of grass were to go on varying, and those varieties were continually selected which differed from eachother in a t all the same manner as distinct species andgenera of grasses differ from each other, a greaternumber of individual plants of this species of grass, including its modified descendants, would succeed in livingon the same piece of ground. And we well know thateach species and each variety of grass is annuallysowing almost countless seeds ; and thus, as it may besaid, is striving its utmost to increase its numbers. Con-

114

NATURAL SELECTION.

CHAP. Iv.

sequently, I cannot doubt that in the course of many
thousands of generations, the most distinct varieties ofany one species of grass would always have the best
chance of succeeding and of increasing in numbers, andthus of supplanting the less distinct varieties; andvarieties, when rendered very distinct from each other,take the rank of species.
The truth of the principle, that the greatest amountof life can be supported by great diversification of
structure, is seen under many natural circumstances.
I n an extremely small area, especially if freely open to
immigration, and where the contest between individual
and individual must be severe, we always find great
diversity in its inhabitants. For instance, I found thata piece of turf, three feet by four in size, which had been
exposed for many years to exactly the same conditions,
supported twenty species of plants, and these belonged
to eighteen genera and to eight orders, which shows how
much these plants differed from each other. So it is withthe plants and insects on small and uniform islets ; andso in small ponds of fresh water. Farmers find that they
can raise most food by a rotation of plants belonging to
the most different orders: nature follows what may becalled a simultaneous rotation. Most of the animals andplants which live close round any small piece of ground,
could live on it (supposing it not to be in any way peculiar
in its nature), and may be said to be striving to the utmost
to live there ; but, it is seen, that where they come into theclosest competition with each other, the advantages ofdiversification of structure, with the accompanying differences of habit and constitution, determine that the
inhabitants, which thus jostle each other most closely,
shall, as a general rule, belong to what we call different
genera and orders.
The same principle is seen in the naturalisation of

CHAP. 1V.

DIVERGENCE OF CHARACTER.

115

plants. through man’s agency in foreign lands. It
might have been expected that the plants which havesucceeded in becoming naturalised in any land would
generally have been closely allied to the indigenes; forthese are commonly looked at as specially created andadapted for their own country. It might, also, perhaps
have been expected that naturalised plants would havebelonged to a few groups more especially adapted tocertain stations in their new homes. But the case isvery different ; and Alph. De Candolle has well remarked
in his great and admirable work, that floras gain bynaturalisation, proportionally with the number of thenat,ive genera and species, far more in new genera thanin new species. To give a single instance: in the lastedition of Dr. Asa Gray’s ‘Manual of the Flora of theNorthern United States,’ 260 naturalised plants are
enumerated, and these belong to 162 genera. We thus
see that these naturalised plants are of a highly diverssed
nature. They differ, moreover, to a large extent fromthe indigenes, for out of the 162 genera, no less than 100
genera are not there indigenous, and thus a large proportional addition is made to the genera of these States.
By considering the nature of the plants or animals
which have struggled successfully with the indigenes ofany country, and have there become naturalised, we
can gain some crude idea in what manner some of thenatives would have had to be modified, in order to havegained an advantage over the other natives; and wemay, I think, at least safely infer that diversification of
structure, amounting to new generic differences, would
have been profitable to them.
The advantage of diversification in the inhabitants ofthe same region is, in fact, the same as that of thephysiological division of labour in the organs of thesame individual body-a subject so well elucidated by

116

NATURAL SELECTION.

CHAP.11'.

?&lne Edwards. No physiologist doubts that a stomach
by being adapted to digest vegetable matter alone, or
flesh alone, draws most nutriment from these substances.
So in the general economy of any land, the more widelyand perfectly the animals and plants are diversified for
different habits of life, so will a greater number ofindividuals be capable of there supporting themselves.
A set of animals, with their organisation but little diversified, could hardly compete with a set more perfectly
diversified in structure. It may be doubted, for instancc,
whether the Australian marsupials, which are divided
into groups differing but little from each other, andfeebly representing, as Mr. Waterhouse and others haveremarked, our carnivorous, ruminant, and rodent mammals, could successfully compete with these well-pronounced orders. I n the Australian mammals, we see
the process of diversification in an early and incomplete
stage of development.
After the foregoing discussion, which ought to havebeen much amplified, we may, I think, assume that themodified descendants of any one species will succeed byso much the better as they become more diversified instructure, and are thus enabled to encroach on places
occupied by other beings. Now let us see how thisprinciple of great benefit being derived from divergenceof character, combined with the principles of natural
selection and of extinction, will tend to act.
The accompanying diagram will aid us in understanding this rather perplexing subject.* Let A to L represent the species of a genus large in its own country;
these species are supposed to resemble each other inunequal degrees, as is so generally the case in nature,
and as is represented in the diagram by the lettersstanding at unequal distances. I have said a large
genus, because we have seen in the second chapter,*[The diagram referred to appears immediately following the index,
on pages 514-515.-E.M.]

CHAP. IV.

DIVERGENCE O F CHARACTER.

117

that on an average more of the species of large genera
vary than of small genera ; and the varying species ofthe large genera present a greater number of varieties.
We have, also, seen that the species, which are thecommonest and the most widely-diffused, vary morethan rare species with restricted ranges. Let (A) be acommon, widely-diffused, and varying species, belonging to a genus large in its own country. The little fanof diverging dotted lines of unequal lengths proceedingfrom (A), may represent its varying offspring. The
variations are supposed to be extremely slight, but of
the most diversified nature ; they are not supposed all
to appear simultaneously, but often after long intervalsof time ; nor are they all supposed to endure for equal
periods. Only those variations which are in some way
profitable will be preserved or naturally selected. And
here the importance of the principle of benefit being
derived from divergence of character comes in ; for thiswill generally lead to the most different or divergent
variations (represented by the outer dotted lines) being
preserved and accumulated by natural selection. Whena dotted line reaches one of the horizontal lines, and isthere marked by a small numbered letter, a sufficientamount of variation is supposed to have been accumulated to have formed a fairly well-marked variety,such as would be thought worthy of record in a systematic work.
The intervals between the horizontal lines in the
diagram, may represent each a thousand generations ;but it would have been better if each had represented
ten thousand generations. After a thousand generations, species (A) is supposed to have produced two
fairly well-marked varieties, namely a’ and m’. These
two varieties will generally continue to be exposed tothe same conditions which made their parents variable,

A

B

C

D

[DIAGRAM OF

EF

.. . .
u7'

(&7.

.

.
. .. .
,:. 7
: ._

w

m
II
I

G

H

I

K

L

DIVERGENCE OF T A U ]118

NATURAL SELECTION.

CHAP. Iv.

and the tendency to variability is in itself hereditary,consequently they will tend to vary, and generally
to vary in nearly the same manner as their parents
varied. Moreover, these two varieties, being only
slightlymodified forms, will tend to inherit those advantages which made their common parent (A) more
numerous than most of the other inhabitants of thesame country ; they will likewise partake of those moregeneral advantages which made the genus to which theparent-species belonged, a large genus in its own country.
And these circumstances we know to be favourable tothe production of new varieties.
If, then, these two varieties be variable, the mostdivergent of their variations will generally be preserved
during the next thousand generations. And after thisinterval, variety a' is supposed in the diagram to haveproduced variety u2, which will, owing to the principleof divergence, differ more from (A) than did variety
a'. Variety m' is supposed to have produced twovarieties, namely m2 and 2,differing from each other,and more considerably from their common parent (A).
We may continue the process by similar steps for any
length of time ; some of the varieties, after each thousand
generations, producing only a single variety, but in amore and more modified condition, some producing twoor three varieties, and some failing to produce any.
Thus the varieties or modified descendants, proceedingfrom the common parent (A), will generally go on increasing in number and diverging in character. I n
the diagram the process is represented up to the tenthousandth generation, and under a condensed and simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose thatthe process ever goes on so regularly as is represented in
the diagram, though in itself made somewhat irregular.

CHAP. IV.

DIVERGENCE O F CHARACTER.

119

I an1 far from thin1:ing that the most divergent varieties
will invariably prevail and multiply: a medium form
may often long endure, and may or may not produce
more than one modified descendant ; for natural selection will always act according to the nature of theplaces which are either unoccupied or not perfectly
occupied by other beings ; and this will depend on infinitely complex relations. But as a general rule, themore diversified in structure the descendants from any
one species can be rendered, the more places they will
be enabled to seize on, and the more their modified
progeny will be increased. I n our diagram the line ofsuccession is broken at regular intervals by small numbered letters marking the successive forms which havebecome sufficiently distinct to be recorded as varieties.
But these breaks are imaginary, and might have been
inserted anywhere, after intervals long enough to haveallowed the accumulation of a considerable amount ofdivergent variation.
As all the modified descendants from a common andwidely-diffused species, belonging to a large genus, willtend to partake of the same advantages which madetheir parent successful in life, they will generally goon multiplying in number as well as diverging incharacter: this is represented in the diagram by theseveral divergent branches proceeding from (A). The
modified offspring from the later and more highly improved branches in the lines of descent, will, it is probable, often take the place of, and so destroy, theearlier and less improved branches : this is represented
in the diagram by some of the lower branches not reaching to the upper horizontal lines. I n some cases I do
not doubt that the process of modification will be confined to a single line of descent, and the number of thedescendants will not be increased ; although the amount120

NATURAL SELECTION.

CHAP. IV.

of divergent modification may have been increased in the
successive generations. This case would be represented
in the diagram, if all the lines proceeding from (A)were removed, excepting that from a‘ to a’O. I n thesame way, for instance, the English race-horse andEnglish pointer have apparently both gone on slowly
diverging in character from their original stocks, without either having given off any fresh branches or races.
After ten thousand generations, species (A) is supposed
to have produced three forms, a lo, f l o , and m lo, which,from having diverged in character during the successive
generations, will have come to differ largely, but perhaps
unequally, from each other and from their common
parent. If we suppose the amount of change between
each horizontal line in our diagram to be excessively
small, these three forms may still be only well-marked
varieties; or they may have arrived at the doubtful
category of sub-species; but we have only to suppose
the steps in the process of modification to be more
numerous or greater in amount, to convert these threeforms into well-defined species : thus the diagram
illustrates the steps by which the small differencesdistinguishing varieties are increased into the larger
differences distinguishing species. By continuing thesame process for a greater number of generations (asshown in the diagram in a condensed and simplified
manner), we get eight species, marked by the letters
between a l 4 and f n I*, all descended from (A). Thus, as I
believe, species are multiplied and genera are formed.
I11 a large genus it is probable that more than one
species would vary. I n the diagram I have assumed
that a second species (I) has produced, by analogous
steps, after ten thousand generations, either two wellmarked varieties (w’”and Z ’ O ) or two species, according
to the amount of change supposed to be represented be-CHAP. IV.

DIVERGENCE OF CHARACTER.

121

tween the horizontal lines. After fourteen thousand
generations, six new species, marked by the letters n l4
to z 14, are supposed to have been produced. I n eachgenus, the species, which are already extremely different in character, will generally tend to produce thegreatest number of modified descendants ; for these will
have the best chance of filling new and widely different
places in the polity of nature: hence in the diagram I
have chosen the extreme species (A), and the nearlyextreme species (I),as those which have largely varied,and have given rise to new varieties and species. The
other nine species (marked by capital letters) of ouroriginal genus, may for a long period continue transmitting unaltered descendants; and this is shown inthe diagram by the dotted lines not prolonged far upwards from want of space.
But during the process of modification, represented
in the diagram, another of our principles, namely thatof extinction, will have played an important part. As ineach fully stocked country natural selection necessarily
acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency in the improved descendants of any one
species to supplant and exterminate in each stage ofdescent their predecessors and their original parent.
For it should be remembered that the competition will
generally be most severe between those forms which
are most nearly related to each other in habits, constitution, and structure. Hence all the intermediate forms
between the earlier and later states, that is between theless and more improved state of a species, as well as
the oriffinal parent-species itself, will generally tend to
become extinct. So it probably will be with many wholecollateral lines of descent, which will be conquered bylater and improved lines of descent. If, however, the122

NATURAL SELECTION.

CHAP.IV.

modified offspring of a species get into some distinct
country, or become quickly adapted to some quite newstation, in which child and parent do not come into
competition, both may continue to exist.
If then our diagram be assumed to represent a
considerable amount of modification, species (A) andall the earlier varieties will have become extinct,
having been replaced by eight new species (a l4 to m 14) ;and (I) will have been replaced by six (n l4 to z 14) new
species.
But we may go further than this. The original speciesof our genus were supposed to resemble each other inunequal degrees, as is so generally the case in nature ;species (A) being more nearly related to B, C, and D,
than to the other species ; and species (I) more to G, H,
I(,L, than to the others. These two species (A) and (I),were also supposed to be very common and widely diffused species, so that they must originally have hadsome advantage over most of the other species of thegenus. Their modified descendants, fourteen in number
at the fourteen-thousandth generation, will probably
have inherited some of the same advantages: they
have also been modified and improved in a diversified
manner at each stage of descent, so as to have become
adapted to many related places in the natural economyof their country. It seems, therefore, to me extremely
probable that they will have taken the places of, andthus exterminated, not only their parents (A) and (I),but likewise some of the original species which weremost nearly related to their parents. Hence very few ofthe original species will have transmitted offspring to thefourteen-thousandth generation. We may suppose that
only one (F), of the two species which were least closely
related to the other nine original species, has transmitted
descendants to this late stage of descent.

CHAP. IV.

DIVERGENCE OF 'CHAMCTER.

123

The new species in our diagram descended from theoriginal eleven species, will now be fifteen in number.
Owing to the divergent tendency of natural selection,
the extreme amount of difference in character between
speciesand z14 will be much greater than thatbetween the most different of the original eleven species.
The new species, moreover, will be allied to each otherin a widely different manner. Of the eight descendants
from (A) the three marked a 14, q 14, p 14, will be nearly
related from having recently branched off from a lo ; b l4
and f 14, from having diverged at an earlier period froma5,will be in some degree distinct from the three firstnamed species; and lastly, o14, el4, and mlS, will benearly related one to the other, but from having diverged at the first commencement of the process ofmodification, will be widely different from the otherfive species, and may constitute a sub-genus or even a
distinct genus.
The six descendants from (I) will form two subgenera or even genera. But as the original species (I)differed largely from (A), standing nearly at the extreme
points of the original genus, the six descendants from(I) will,owing to inheritance, differ considerably fromthe eight descendants from (A) ; the two groups, moreover, are supposed to have gone on diverging in different directions. The intermediate species, also (andthis is a very important consideration), which connected
the original species (A) and (I), have all become, excepting (F), extinct, and have left no descendants.
Hence the six new species descended from (I),and theeight descended from (A), will have to be ranked asvery distinct genera, or even as distinct sub-families.
Thus it is, as I believe, that two or more genera
are produced by descent, with modification, from two
or more species of the same genus. And the two or

124

NATURAL SELECTION.

CHAP.IV.

more parent-species are supposed to have descended
from some one species of an earlier genus. In ourdiagram, this is indicated by the broken lines, beneaththe capital letters, converging in sub-branches downwards towards a single point; this point representing
a single species, the supposed single parent of our
several new sub-genera and genera.
It is worth while to reflect for a moment on the character of the new species F 14, which is supposed not tohave diverged much in character, but to have retained
the form of (F), either unaltered or altered only in aslight degree. In this case, its affinities to the otherfourteen new species will be of a curious and circuitous
nature. Having descended from a form which stood
between the two parent-species (A) and (I), now s u p
posed to be extinct and unknown, it wiU be in somedegree intermediate in character between the twogroups descended from these species. But as these twogroups have gone on diverging in character from thetype of their parents, the new species (FI4) will not bedirectly intermediate between them, but rather between
types of the two groups; and every naturalist will beable to bring some such case before his mind.
I n the diagram, each horizontal line has hitherto been
supposed to represent a thousand generations, but eachmay represent a million or hundred million generations,and likewise a section of t.he successive strata of theearth's crust including extinct remains. We shall, when
we come to our chapter on Geology, have to refer again
to this subject, and I think we shall then see that the
diagram throws light on the &hities of extinct beings,
which, though generally belonging to the same orders,or families, or genera, with those now living, yet areoften, in some degree, intermediate in character between
existing groups; and we can understand this fact, forCHAP. IV.

QIVERGIENCE O F CHARACTER.

125

the extinct species lived at very ancient epochs when
the branching lines of descent had diverged less.
I see no reason to limit the process of modification, as
now explained, to the formation of genera alone. If, in
our diagram, we suppose the amount of change represented by each successive group of diverging dotted lines
to be very great, the forms marked a l4 to p l', thosemarked b l4 andf'*, and those marked o l 4 to m 14, will
form three very distinct genera. We shall also have twovery distinct genera descended fiom (I); and as theselatter two genera, both from continued divergence ofcharacter and from inheritance from a different parent,
w i l l differ widely from the three genera descended from(A), the two little groups of genera will form two distinct
families, or even orders, according to the amount ofdivergent modification supposed to be represented in the
diagram. And the two new families, or orders, will havedescended from two species of the original genus ; andthese two species are supposed to have descended fromone species of a still more ancient and unknown genus.
We have seen that in each country it is the speciesof the larger genera which oftenest present varieties orincipient species. This, indeed, might have been expected ; for as natural selection acts through one form
having some advantage over other forms in the struggle
for existence, it will chiefly act on those which alreadyhave some advantage ; and the largeness of any groupshows that its species have inherited from a common
ancestor some advantage in common. Hence, thestruggle for the production of new and modified descendants, will mainly lie between the larger groups,
which are all trying to increase in number. One
large group will slowly conquer another large group,
reduce its numbers, and thus lessen its chance of further
variation and improvement. Within the same large

126

NATURAL SELECTION.

CHAP. IV.

group, the later and more highly perfected sub-groups,
from branching out and seizing on many new placesin the polity of Nature, will constantly tend to supplantand destroy the earlier and less improved sub-groups.
Small and broken groups and sub-groups will finallytend to disappear. Looking to the future, we can predict that the groups of organic beings which are now
large and triumphant, and which are least broken up,
that is, which as yet have suffered least extinction,
will for a long period continue to increase. But whichgroups will ultimately prevail, no man can predict;for we well know that many groups, formerly mostextensively developed, have now become extinct. Looking still more remotely to the future, we may predict
that, owing to the continued and steady increase of thelarger groups, a multitude of smaller groups will become
utterly extinct, and leave no modified descendants ; andconsequently that of the species living at any one period,
extremely few will transmit descendants to a remotefuturity. I shall have to return to this subject in thechapter on Classification, but I may add that on thisview of extremely few of the more ancient species
having transmitted descendants, and on the view of all
the descendants of the same species making a class,
we can understand how it is that there exist but very
few classes in each main division of the animal andvegetable kingdoms. Although extremely few of themost ancient species may now have living and modxed
descendants, yet at the most remate geological period,
the earth may have been as well peopled with many
species of many genera, families, orders, and classes, asat the present day.
Summary of Chapter.-If during the long course ofages and under varying conditions of life, organic beings

CHAP. IV.

SUMMARY.

127

vary at all in the several parts of their organisation, andI think this cannot be disputed ; if there be, owing to thehigh geometrical powers of increase of each species, atsome age, season, or year, a severe struggle for life, andthis certainly cannot be disputed ; then, considering theinfinite complexity of the relations of all organic beings
to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, andhabits, to be advantageous to them, I think it would
be a most extraordinary fact if no variation ever had
occimed useful to each being's own welfare, in the same
way as so many variations have occurred useful to man.
But if variations useful to any organic being do occur,
assuredly individuals thus characterised will have thebest chmce of being preserved in the struggle for life ;and from the strong principle of inheritance they willtend to produce offspring similarly characterised. This
principle of preservation, I have called, for the sakeof brevity, Natural Selection. Natural selection, on theprinciple of qualities being inherited at corresponding
ages, can modify the egg, seed, or young, as easily as
the adult. Amongst many animals, sexual selection
will give its aid to ordinary selection, by assuring to themost vigorous and best adapted males the greatest
number of offspring. Sexual selection will also give
characters useful to the males alone, in their struggles
with other males.
Whether natural selection has really thus acted innature, in modifying and adapting the various forms oflife to their several conditions and stations, must be
judged of by the general tenour and balance of evidence
given in the following chapters. But we already see
how it entails extinction ; and how largely extinction
has acted in the world's history, geology plainly declares. Natural selection, also, leads to divergence of

128

NATURAL SELECTION.

CHAP. IV.

character ; for more living beings can be supported on
the same area the more they diverge in structure, habits,
and constitution, of which we see proof by looking at
the inhabitants of any small spot or at naturalised productions. Therefore during the modification of thedescendants of any one species, and during the incessantstruggle of all species to increase in numbers, the morediversified these descendants become, the better will betheir chance of succeeding in the battle of life. Thus
the small differences distinguishing varieties of the same
species, will steadily tend to increase till they come toequal the greater differences between species of thesame genus, or even of distinct genera.
We have seen that it is the common, the widelydiffused, and widely-ranging species, belonging to thelarger genera, which vary most ; and these will tend totransmit to their modified offspring that superiority
which now makes them dominant in their own countries. Natural selection, as has just been remarked,
leads to divergence of character and to much extinctionof the less improved and intermediate forms of life. On
these principles, I believe, the nature of the affinitiesof all organic beings may be explained. It is a truly
wonderful fact-the wonder of which we are apt tooverlook from familiarity-that
all animals and allplants throughout all time and space should be related
to each other in group subordinate t6’group, in themanner which we everywhere behofd-namely, varieties of the same species most closely related together,
species of the same genus less closely and unequallyrelated together, forming sections and sub-genera, species of distinct genera much less closely related, andgenera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. The
several subordinate groups in any class cannot be

CHaP. IV.

SUMMARY.

129

ranked in a single fle, but seem rather to be clustered
round points, and these round other points, and so on
in almost endless cycles. On the view that each species has been independently created, I can see no
explanation of this great fact in the classification of allorganic beings ; but, to the best of my judgment, it isexplained through inheritance and the complex actionof natural selection, entailing extinction and divergenceof character, as we have seen illustrated in the diagram.
The affinities of all the beings of the same class havesometimes been represented by a great tree. I believe
this simile largely speaks the truth. The green andbudding twigs may represent existing species ; and those
produced during each former year may represent thelong succession of extinct species. At each period of
growth all the growing twigs have tried to branch out
on all sides, and to overtop and kill the surrounding
twigs and branches, in the same manner as species andgroups of species have tried to overmaster other species
in the great battle for life. The limbs divided into
great branches, and these into lesser and lesser branches,
were themselves once, when the tree was small, budding
twigs; and this connexion of the former and present
buds by ramifying branches may well represent theclassification of all extinct and living species in groups
subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or
three, now grown into great branches, yet survive andbear all the other branches ; so with the species which
lived during long-past geological periods, very few now
have living and modified descendants. From the firstgrowth of the tree, many a limb and branch has decayed
and dropped off; and these lost branches of various
sizes may represent those whole orders, families, andgenera which have now no living representatives, and130

NATURAL SELECTION.

CHAP.IV.

which are known to us only from having been found ina fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree,
and which by some chance has been favoured and is
still alive on its summit, so we occasionally see ananimal like the Ornithorhynchus or Lepidosiren, which
in some small degree connects by its affinities two large
branches of life, and which has apparently been saved
from fatal competition by having inhabited a protected
station. As buds give rise by growth to fresh buds, andthese, if vigorous, branch out and overtop on all sides
many a feebler branch, so by generation I believe it
has been with the great Tree of Life, which fills withits dead and broken branches the crust of the earth,and covers the surface with its ever branching andbeautifid ramifications.

CHAP. V.

LAWS OF VARIATION.

131

CHAPTER V.
LAWSOF VARIATION.

-

Effects of external conditions
Use and disuse, combined withnatural selection ; organs of flight and of vision-Acclimatisation -Correlation of growth - Compensation and economy ofgrowth-False correlations-Multiple, rudimentary, and lowly
organised structures variable-Parts developed in an unusual
manner are highly variable : specific characters more variable
than generic : secondary sexual characters variable-Species ofthe same genus vary in an analogous manner-Reversions tolong lost characters-Summary.

I HAVE hitherto sometimes spoken as if the variations-so common and multiform in organic beings underdomestication, and in a lesser degree in those in a stateof nature-had been due to chance. This, of course, isa wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of.each particular variation, Some authors believe it to be as muchthe function of the reproductive system to produce
individual differences, or very slight deviations of structure, as to make the child like its parents. But themuch greater variability, as Fell as the greater frequency of monstrosities, under domestication or cultivation, than under nature, leads me to believe thatdeviations of structure are in some way due to thenature of the conditions of life, to which the parents
and their more remote ancestors have been exposed
during several generations. I have remarked in thefirst chapter-but a long catalogue of facts which cannot be here given would be necessary to show the truth of
the remark-that the reproductive system is eminently
susceptible to changes in the conditions of life ; and to132

LAWS OF VARIATION.

CHAP. v.

this system being functionally disturbed in the parents,
I chiefly attribute the varying or plastic condition of
the offspring. The male and female sexual elements
seem to be affected before that union takes place whichis to form a new being. I n the case of “sporting”
plants, the bud, which in its earliest condition does notapparently differ essentially from an ovule, is alone
affected. But why, because the reproductive system isdisturbed, this or that part should vary more or less, weare profoundly ignorant. Nevertheless, we can here
and there dimly catch a faint ra.y of light, and we
may feel sure that there must be some cause for eachdeviation of structure, however slight.
How much direct effect difference of climate, food,&c., produces on any being is extremely doubtful. My
impression is, that the effect is extremely small in thew e of animals, but perhaps rather more in that ofplants. We may, at least, safely conclude that suchinfluences cannot have produced the many striking
and complex co-adaptations of structure between one
organic being and another, which we see everywhere
throughout nature. Some little influence may be attributed to climate, food, &c.: thus, E. Forbes speaks
confidently that shells at their southern limit, and whenliving in shallow water, are more brightly coloured thanthose of the same species further north or from greater
depths. Gould believes that birds of the same species
are more brightly coloured under a clear atmosphere,
than when living on islanda or near the coast. So with
insects, Wollaston is convinced that residence near thesea affects their colours. Moquin-Tandon gives a listof plants which when growing near the sea-shore havetheir leaves in some degree fleshy, though not elsewhere
fleshy. Several other such cases could be given.
The fact of varieties of one species, when they range

CHAP. V.

LAWS OF VARIATION.

133

into the zone of habitation of other species, often
acquiring in a very slight degree some of the charact,ers
of such species, accords with our view that species ofall kinds are only well-marked and permanent varieties.
Thus the species of shells which are confined to tropicaland shallow seas are generally brighter-coloured thanthose confined to cold and deeper seas. The birds
which are confined to continents are, according to Mr.
Gould, brighter-coloured than those of islands. The
insect-species confined to sea-coasts, as every collector
knows, are often brassy or lurid, Plants which live
exclusively on the sea-side are very apt to have fleshy
leaves. He who believes in the creation of each species, will have to say that this shell,for instance, wascreated with bright colours for a warm sea; but that
this other shell became bright-coloured by variation
when it ranged into warmer or shallower waters.
When a variation is of the slightest use to a being,
we cannot tell how much of it to attribute to the accumulative action of natural selection, and how much tothe conditions of life. Thus, it is well known to furriers that animals of the same species have thicker andbetter fur the more severe the climate is under which
they, have lived ; but who can tell how much of thisdifference may be due to the warmest-clad individuals
having been favoured and preserved during many
generations, and how much to the direct action of
the severe climate? for it would appear that climate
has some direct action on the hair of our domestic
quadrupeds.
Instances could be given of the same variety being
produced under conditions of life as different as canwell be conceived ; and, on the other hand, of different
varieties being produced from the same species under
the same conditions. Such facts show how indirectly

134

LAWS OF VARIATION.

CHAP. V.

the conditions of life must act. Again, innumerable
instances are known to every naturalist of species
keeping true, or not varying at all, although living
under the most opposite climates. Such considerations
as these incline me to lay very little weight on the
direct action of the conditions of life. Indirectly, as
already remarked, they seem to play an important partin affecting the reproductive system, and in thus inducing variability ; and natural selection will then accumulate all profitable variations, however slight, until
they become plainly developed and appreciable by us.

Xfects of Use and Disuse.-From the facts alluded toin the first chapter, I think there can be little doubtthat use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them ; andthat such modifications are inherited. Under freenature, we can have no standard of comparison, bywhich to judge of the effects of long-continued use ordisuse, for we know not the parent-forms; but many
animals have structures which can be explained by theeffects of disuse. As Professor Owen has remarked, there
is no great,er anomaly in nature than a bird that cannot
fly; yet there are several in this state. The loggerheaded duck of South Amerka can only flap along thesurface of the water, and has its wings in nearly thesame condition as the domestic Aylesbury duck. As thelarger ground-feeding birds seldom take flight except toescape danger, I believe that the nearly wingless condition of several birds, which now inhabit or have lately
inhabited several oceanic islands, tenanted by no beastof prey, has been caused by disuse. The ostrich indeed
inhabits continents and is exposed to danger from which
it cannot escape by flight, but by kicking it can defend
itself from enemies, as well aa any of the smallerCHAP. V.

USE AND DISUSE.

135

quadrupeds. We may imagine that the early progenitor
of the ostrich had habits like those of a bustard, andthat as natural selection increased in successive generations the size and weight of its body, its legs were used
more, and its wings less, until they became incapableof flight.
Kirby has remarked (and I have observed the samefact) that the anterior tarsi, or feet, of many male dungfeeding beetles are very often broken off; he examined
seventeen specimens in his own collection, and not one
had even a relic left. In the Onites apelles the tarsi
are so habitually lost, that the insect has been describedas not having them. In some other genera they are present, but in a rudimentary condition. I n the Ateuchus
or sacred beetle of the Egyptians, they are totally deficient. There is not suEcient evidence to induce us
to believe that mutilations are ever inherited; and I
should prefer explaining the entire absence of the anterior tarsi in Ateuchus, and their rudimentary condit,ion
in some other genera, by the long-continued effects ofdisuse in their progenitors ; for as the tarsi are almost
always lost in many dung-feeding beetles, they must
be lost early in life, and therefore cannot be much usedby these insects.
I n some cases we might easily put down to disuse
modifications of structure which are wholly, or mainly,
due to natural selection. Mr. Wollaston has discovered
the remarkable fact that 200 beetles, out of the 550
species inhabiting Madeira, are so far deficient in wings
that they cannot fly; and that of the twenty-nine
endemic genera, no less than twenty-three genera have
all their species in this condition ! Several facts, namely,that beetles in many parts of the world are very frequently blown to sea and perish; that the beetles in
Madeira, as observed by Mr. Wollaston, lie much con-

136

LAWS OF VARIATION.

CHAP.

v.

ceded, until the wind lulls and the sun shines; thatthe proportion of wingless beetles is larger on the exposed Dezertas than in Madeira itself; and especiallythe extraordinary fact, so strongly insisted on by Mr.
Wollaston, of the almost entire absence of certain large
groups of beetles, elsewhere excessively numerous, andwhich groups have habits of life almost necessitating
frequent flight ;-these several considerations have mademe believe that the wingless condition of so many
Madeira beetles is mainly due to the action of natural
selection, but combined probably with disuse. For
during thousands of successive generations each individual beetle which flew least, either from its wings
having been ever so little less perfectly developed orfrom indolent habit, will have had the best chance ofsurviving from not being blown out to sea ; and, on theother hand, those beetles which most readily took toflight will oftenest have been blown to sea and thus
have been destroyed.
The insects in Madeira which are not ground-feeders,
and which, as the flower-feeding coleoptera and lepidoptera, must habitually use their wings to gain their subsistence, have, as Mr. Wollaston suspects, their wings nota t all reduced, but even enlarged. This is quite compatible with the action of natural selection. For whena new insect first arrived on the island, the tendencyof natural selection to enlarge or to reduce the wings,
would depend on whether a greater number of individuals were saved by successfully battling with the winds,
or by giving up the attempt and rarely or never flying.
As with mariners shipwrecked near a coast, it would
have been better for the good swimmers if they had
been able to swim still further, whereas it would havebeen better for the bad swimmers if they had not beenable to swim at all and had stuck to the wreck.

CHAP. V.

U S I AND DISUSE.

137

The eyes of moles and of some burrowing rodents are
rudimentary in size, and in some cases are quite covered
up by skin and fur. This state of the eyes is probably
due to gradual reduction from disuse, but aided perhaps
by natural selection. In South America, a burrowing
rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its habits than the mole ; and I was assured
by a Spaniard, who had often caught them, that thsy
were frequently blind ; one which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating
membrane. As frequent inflammation of the eyes must
be injurious to any animal, and as eyes are certainly
not indispensable to animals with subterranean habits,
a reduction in their size with the adhesion of the eyelids and growth of fur over them, might in such case be
an advantage; and if so, natural selection would constantly aid the effects of disuse.
It is well known that several animals, belonging to themost different classes, which inhabit the caves of Styria
and of Kentucky, are blind. I n some of the crabs the
foot-stalk for the eye remains, though the eye is gone ;the stand for the telescope is there, though the telescopewith its glasses has been lost. As it is difficult to imagine that eyes, t-hough useless, could be in any way
injurious to animals living in darkness, I attribute theirloss wholly to disuse. I n one of the blind animals,
namely, the cave-rat, the eyes are of immense size ; andProfessor Silliman thought that it regained, after living
some days in the light, some slight power of vision. In
the same manner as in Madeira the wings of some of
the insects have been enlarged, and the wings of others
have been reduced by natural selection aided by use
and disuse, so in the case of the cave-rat natural selection seems to have struggled with the loss of light and138

LAWS OF VARIATION.

CHAP.

v.

to have increased the size of the eyes; whereas withall the other inhabitants of the caves, disuse by itself
seems to have done its work.
It is difficult to imagine conditions of life more similar
than deep limestone caverns under a nearly similar
climate ; so that on the common view of the blind animals having been separately created for the Americanand European caverns, close similarity in their organisation and affinities might have been expected ; but, as
Schiodte and others have remarked, this is not the case,and the cave-insects of the two continents are not moreclosely allied than might have been anticipated from thegeneral resemblance of the other inhalitants of North
America and Europe. On my view we must supposethat American animals, having ordinary powers ofvision, slowly migrated by successive generations fromthe outer world into the deeper and deeper recesses of
the Kentucky caves, as did European animals into thecaves of Europe. We have some evidence of this gradation of habit ; for, as Schiodte remarks, “animals
not far remote from ordinary forms, prepare the transition from light to darkness. Next follow those that are
constructed for twilight ; and, last of all, those destinedfor total darkness.” By the time that an animal had
reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection willoften have effected other changes, such as an increasein the length of the antennae or palpi, as a compensation for blindness. Notwithstanding such modifications,
we might expect still to see in the cave-animals of
America, affinities to the other inhabitants of that continent, and in those of Europe, to the inhabitants ofthe European continent. And this is the case withsome of the American cave-animals, as I hear fromCHAP.

v.

ACCLIMATISATION.

139

Professor Dana; and some of the European caveinsects are very closely allied to those of the surrounding country. I t would be most difficult to give any
rational explanation of the affinities of the blind caveanimals to the other inhabitants of the two continents
on the ordinary view of their independent creation.
That several of the inhabitants of the caves of the
Old and New Worlds should be closely related, wemight expect from the well-known relationship of mostof their other productions. Far from feeling any surprise that some of the cave-animals should be very
anomalous, as Agassiz has remarked in regard to theblind fish, the Amblyopsis, and as is the case with theblind Proteus with reference to the reptiles of Europe,
I am only surprised that more wrecks of ancient life
have not been preserved, owing to the less severe competition to which t,he inhabitants of these dark abodes
will probably have been exposed.
Acclimatisation.-Habit is hereditary with plants, asin the period of flowering, in the amount of rain requisite for seeds to germinate, in the time of sleep, &c.,
and this leads me to say a few words on acclimatisation. As it is extremely common for species of thesame genus to inhabit very hot and very cold countries,
and as I believe that all the species of the same genus
have descended from a single parent, if this view becorrect, acclimatisation must be readily effected duringlong-continued descent. It is notorious that eachspecies is adapted to the climate of its own home:
species from an arctic or even from a temperate region
cannot endure a tropical climate, or conversely. So
again, many succulent plants cannot endure a damp
climate. But the degree of adaptation of species tothe climates under which they live is often overrated.

140

LAWS OF VARIATION.

CHAP. V.

W e may infer this from our frequent inability to predict whether or not an imported plant will endure ourclimate, and from the number of plants and animals
brought from warmer countries which here enjoy good
health. We have reason to believe that species in astate of nature are limited in their ranges by the competition of other organic beings quite m much as, ormore than, by adaptation to particular climates. But
whether or not the adaptation be generally very close, wehave evidence, in the case of some few plants, of their
becoming, to a certain extent, naturally habituated todifferent temperatures, or becoming acclimatised : thus
the pines and rhododendrons, raised from seed collectedby Dr. Hooker from trees growing at different heights
on the Himalaya, were found in this country to possess
different constitutimal powers of resjsting cold. Mr.
Thwaites informs me that he has observed similar facts
in Ceylon, and analogous observations have been madeby Mr. H. C. Watson on European species of plants
brought from the Azores to England. I n regard toanimals, several authentic cases could be given of
species witEhinhistorical times having largely extended
their range from warmer to cooler latitudes, and conversely ; but we do not positively know that these animals were strictly adapted to their native climate, but
in all ordinary cases we assume such to be the case;nor do we know that they have subsequently become
acclimatised to their new homes.
As I believe that our domestic animals were originally chosen by uncivilised man because they were useful and bred readily under confinement, and not because
they were subsequently found capable of far-extended
transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most different climates but of being perfectly

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ACCLIMATISATION.

141

fertile (a far severer test) under them, may be ysed as
an argument that a large proportion of other animals,
now in a state of nature, could easily be brought to bear
widely different climates. We must not, however, push
the foregoing argument too far, on account of the probable origin of some of our domestic animals from several wild stocks: the blood, for instance, of a tropicaland arctic wolf or wild dog may perhaps be mingled inour domestic breeds. The rat and mouse cannot be
considered as domestic animals, but they have beentransported by man to many parts of the world, andnow have a far wider range than any other rodent,
living free under the cold climate of Faroe in thenorth and of the Falklands in the south, and on manyislands in the torrid zones. Hence I am inclined to lookat adaptation to any special climate as a quality readily
grafted on an innate wide flexibility of constitution,
which is common to most animals. On this view, thecapacity of enduring the most different climates by man
himself and by his domestic animals, and such facts asthat former species of the elephant and rhinoceros werecapable of enduring a glacial climate, whereas the living species are now all tropical or sub-tropical in theirhabits, ought not to be looked at as anomalies, but
merely as examples of a very common flexibility of constitution, brought, under peculiar circumstances, into
PlayHow much of the acclimatisation of species to any
peculiar climate is due to mere habit, and how much to
the natural selection of varieties having different innate
constitutions, and how much to both means combined, isa very obscure question. That habit or custom has someinfluence I must believe, both from analogy, and fromthe incessant advice given in agricultural works, evenin the ancient Encyclopaedias of China, to be very cau-

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LAW8 OF VARIATION.

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v.

tious in transposing animals from one district to another; for it is not likely that man should have succeeded in selecting so many breeds and sub-breeds withconstitutions specially fitted for their own districts : theresult must, I think, be due to habit. On the other
hand, I can see no reason to doubt that natural selection
will continually tend to preserve those individuals which
are born with constitutions best adapted to their native
countries. I n treatises on many kinds of cultivatedplants, certain varieties are said to withstand certain
climates better than others: this is very strikingly
shown in works on fruit trees published in the UnitedStates, in which certain varieties are habitually recommended for the northern, and others for the southern
States; and as most of these varieties are of recent
origin, they cannot owe their constitutional differences
to habit. The case of the Jerusalem artichoke, which
is never propagated by seed, and of which consequently
new varieties have not been produced, has even been
advanced-for it is now as tender as ever it was-as
proving that acclimatisation cannot be effected ! The
case, also, of the kidney-bean has been often cited for asimilar purpose, and with much greater weight; but
until some one will sow, during a score of generations,his kidney-beans so early that a very large proportion
are destroyed by frost, and then collect seed from the
few survivors, with care to prevent accidental crosses,
and then again get seed from these seedlings, with thesame precautions, the experiment cannot be said tohave been even tried. Nor let it be supposed that nodifferences in the constitution of seedling kidney-beansever appear, for an account has been published how
much more hardy some seedlings appeared to be than
0th ers.
On the whole, I think we may conclude that habit,

CHAP.

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CORRELATION OF GROWTH.

143

use, and disuse, have, in some cases, played a considerable part in the modification of the constitution, andof the structure of various organs ; but that the effects
of use and disuse have often been largely combined
with, and sometimes overmastered by, the natural selection of innate differences.
Correlation of Growth.-I
mean by this expressionthat the whole organisation is so tied together during its
growth and development, that when slight variations in
any one part occur, and are accumulated through natural
selection, other parts become modified. This is a very
important subject, most imperfcctly understood. The
most obvious case is, that modifications accumulated
solely for the good of the young or larva, will, it may
safely be concluded, affect the structure of the adult;
in the same manner as any malconformation affecting
the early embryo, seriously affects the whole organisation of the adult. The several parts of the body which
are homologous, and which, at an early embryonic period,
are alike, seem liable to vary in an allied manner: we
see this in the right and left sides of the body varying
in the same manner; in the front and hind legs, andeven in the jaws and limbs, varying together, for the
lower jaw is believed to be homologous with the limbs.
These tendencies, I do not doubt, may be mastered
more or less completely by natural selection: thus afamily of stags once existed with an antler only on
one side ; and if this had been of any great use to thebreed it might probably have been rendered permanent
by natural selection.
Homologous parts, as has been remarked by someauthors, tend to cohere ; this is often seen in monstrous
plants ; and nothing is more common than the union ofhomologous parts in normal structures, as the union of

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LAWS OF VARIATION.

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the petals of the corolla into a tube. Hard parts seemto affect the form of adjoining soft parts ; it is believedby some authors that the diversity in the shape of thepelvis in birds causes the remarkable diversity in the
shape of their kidneys. Others believe that the shapeof the pelvis in the human mother influences by pressure the shape of the head of the child. I n snakes,
according to Schlegel, the shape of the body and themanner of swallowing determine the position of severalof the most important viscera.
The nature of the bond of correlation is very frequently quite obscure. M. Is. Geoffroy St. Hilaire has
forcibly remarked, that certain malconformations veryfrequently, and that others rarely coexist, without ourbeing able to assign any reason. What can be more
singular than the relation between blue eyes and deafness in cats, and the tortoise-shell colour with the female
sex ; the feathered feet and skin between the outer toesin pigeons, and the presence of more or less down on theyoung birds when first hatched, with the future colour of
their plumage ; or, again, the relation between the hairand teeth in the naked Turkish dog, though here probably homology comes into play? With respect to thislatter case of correlation, I think it can hardly be accidental, that if we pick out the two orders of mammalia
which are most abnormal in their dermal covering, via
Cetacea (whales) and Edentata (armadilloes, scaly anteaters, &.), that these are likewise the most abnormal
in their teeth.
I know of no case better adapted to show the importance of the laws of correlation in modifying important structures, independently of utility and, therefore, of natural selection, than that of the difference
between the outer and inner flowers in some Compoaitous and Umbelliferous plants. Every one knows theCHAP.V.

CORRELATION OF GROWTH.

145

difference in the ray and central florets of, for instance,
the daisy, and this difference is often accompanied withthe abortion of parts of the flower. But, in some Compositous plants, the seeds also differ in shape and sculpture ; and even the ovary itself, with its accessory parts,
differs, as has been described by Cassini. These differences have been attributed by some authors to pressure,and the shape of the see& in the ray-florets in some
Compositae countenances this idea ; but, in the case of
the corolla of the Umbelliferae, it is by no means, as Dr.
Hooker idorms me, in species with the densest heads
that the inner and outer flowers most frequently differ.
It might have been thought that the development of
the ray-petals by drawing nourishment from certain
other parts of the flower had caused their abortion ; but
in some Compositae there is a difference in the seeds ofthe outer and inner florets without any difference in thecorolla. Possibly, these several differences may be connected with some difference in the flow of nutrimenttowards the central and external flowers: we know,
at least, that in irregular flowers, those nearest to
the axis are oftenest subject to peloria, and become
regular. I may add, as an instance of this, and of a
striking case of correlation, that I have recently observedin some garden pelargoniums, that the central flower of
the truss often loses the patches of darker colour in the
two upper petals ; and that when this occurs, the adherent nectary is quite aborted ; when the colour is absent
from only one of the two upper petals, the nectary is
only much shortened.
With respect to the difference in the corolla of thecentral and exterior flowers of a head or umbel, I do
not feel at all sure that C. C. Sprengel's idea that the
ray-florets serve to attract insects, whose agency ishighly advantageous in the fertilisation of plants of

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LAWS OF VARIATION.

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v.

these two orders, is so far-fetched, as it may at first
appear: and if it be advantageous, natural selection
may have come into play. But in regard to the differences both in the internal and external structure of theseeds, which are not always correlated with any differences in the flowers, it seems impossible that they canbe in any way advantageous to the plant: yet in the
Umbelliferae these differences are of such apparent importance-the seeds being in some cases, according to
Tausch, orthospermous in the exterior flowers and ccelospermous in the central flowers,-that the elder De
Candolle founded his main divisions of the order on
analogous differences. Hence we see that modifications of structure, viewed by systematists as of highvalue, may be wholly due to unknown laws of correlated
growth, and without being, as far as we can see, of theslightest service to the species.
We may often falsely attribute to correlation of growth,
structures which are common to whole groups of species,
and which in truth are simply due to inheritance;
for an ancient progenitor may have acquired throughnatural selection some one modification in structure,and, after thousands of generations, some other and independent modification ; and these two modifications,having been transmitted to a whole group of descendants
with diverse habits, would naturally be thought to becorrelated in some necessary manner. So, again, I do
not doubt that some apparent correlations, occurring
throughout whole orders, are entirely due to the manneralone in which natural selection can act. For instance,
Alph. De Candolle has remarked that winged seeds arenever found in fruits which do not open: I should explain the rule by the fact that seeds could not gradually
become winged through natural selection, except in fruits
which opened ; so that the individual plants producing

CHAP.V.

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147

seeds which were a little better fitted to be wafted
further, might get an advantage over those producing
seed less fitted for dispersal ; and this process could not
possibly go on in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about
the same period, their law of compensation or balancement of growth; or, as Goethe expressed it, “in orderto spend on one side, nature is forced to economise on
the other side.” I think this holds true to a certain extent with our domestic productions : if nourishment flows
to one part or organ in excess, it rarely flows, at least inexcess, to another part ; thus it is difficult to get a cow
to give much milk and to fatten readily. The same varieties of the cabbage do not yield abundant and nutritious foliage and a copious supply of oil-bearing seeds.
When the seeds in our fruits become atrophied, the fruit
itself gains largely in size and quality, I n our poultry,a large tuft of feathers on the head is generally accompanied by a diminished comb, and a large beard bydiminished wattles. With species in a state of nature
it can hardly be maintained that the law is of.universa1
application ; but many good observers, more especiallybotanists, believe in its truth. I d l not, however, here
give any instances, for I see hardly any way of distinguishing between the effects, on the one hand, of a partbeing largely developed through natural selection andanother and adjoining part being reduced by this sameprocess or by disuse, and, on the other hand, the actual
withdrawal of nutriment from one part owing to theexcess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation
which have been advanced, and likewise some otherfacts, may be merged under a more general principle,
namely, that natural selection is continually trying toeconomise in every part of the organisation. If under

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LAWS OF VARIATION.

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changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its
development, will be seized on by natural selection, forit will profit the individual not to have its nutrimentwasted in building up an useless structure. I can thus
only understand a fact with which I was much stnickwhen examining cirripedes, and of which many otherinstances could be given : namely, that when a cirripede
is parasitic within another and is thus protected, it loses
more or less completely its own shell or carapace. Thisis the case with the male Ibla, and in a truly extraordinary manner with the Proteolepas : for the carapace inall other cirripedes consists of the three highly-important
anterior segments of the head enormously developed,
and furnished with great nerves and muscles; but inthe parasitic and protected Proteolepas, the whole anterior part of the head is reduced to the merest rudimentattached to the bases of the prehensile antennae. Now
the saving of a large and complex structure, when rendered superfluous by the parasitic habits of the Proteolepas, though effected by slow steps, would be a decided
advantage to each successive individual of the species ;for in the struggle for life to which every animal is exposed, each individual Proteolepas would have a betterchance of supporting itself, by less nutriment being
wasted in developing a structure now become useless.
Thus, as I believe, natural selection will always succeed in the long run in reducing and saving every partof the organisation, as soon as it is rendered superfluous,
without by any means causing some other part to belargely developed in a corresponding degree. And, conversely, that natural selection may perfectly well succeed in largely developing any organ, without requiring
as a necessary compensation the reduction of some adjoining part.

CHAP.

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CORRELATION OF GROWTH.

149

It seems to be a rule, as remarked by Is. Geoffroy
St. Hilaire, both in varieties and in species, that when
any part or organ is repeated many times in the structure of the same individual (as the vertebrz in snakes,
and the stamens in polyandrous flowers) the number isvariable ; whereas the number of the same part or organ,
when it occurs in lesser numbers, is constant. The sameauthor and some botanists have further remarked thatmultiple parts are also very liable to variation in sbructure. Inasmuch as this “ vegetative repetition,” to use
Prof. Owen’s expression, seems to be a sign of low organisation ; the foregoing remark seems connected with thevery general opinion of naturalists, that beings low inthe scale of nature are more variable than those which
are higher. I presume that lowness in this case meansthat the several parts of the organisation have been but
little specialised for particular functions ; and as long as
the same part has to perform diversified work, we canperhaps see why it should remain variable, that is, why
natural selection should have preserved or rejected eachlittle deviation of form less carefully than when the parthas to serve for one special purpose alone. I n the same
way that a knife which has to cut all sorts of things
may be of almost any shape; whilst a tool for someparticular object had better be of some particular shape.
Natural selection, it should never be forgotten, can act
on each part of each being, solely through and for its
advantage.
Rudimentary parts, it has been stated by someauthors, and I believe with truth, are apt to be highlyvariable. We shall have to recur to the general subjectof rudimentary and aborted organs ; and I will here only
add that their variability seems to be owing to theiruselessness, and therefore to natural selection having
no power to check deviations in their structure. Thus

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LAWS OF VARIATION.

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v.

rudimentary parts are left to the free play of the various laws of growth, to the effects of long-continued disuse, and to the tendency to reversion.

A part developed in any species in an extraordinary
degree or manner, in comparison with the same part inallied species, tends to be highly variable.-Several years
ago I was much struck with a remark, nearly to theabove effect, published by Mr. Waterhouse. I inferalso from an observation made by Professor Owen, withrespect to the length of the arms of the ourang-outang,that he has come to a nearly similar conclusion. It is
hopeless to attempt to convince any one of the truth of
this proposition without giving the long array of facts
which I have collected, and which cannot possibly behere introduced. I can only state my conviction that it isa rule of high generality. I am aware of several causesof error, but I hope that I have made due allowance forthem. It should be understood that the rule by nomeans applies to any part, however unusually developed,
unless it be unusually developed in comparison withthe same part in closely allied species. Thus, the bat’s
wing is a most abnormal structure in the class mammalia ; but the rule would not here apply, because there
is a whole group of bats having wings; it would apply
only if some one species of bat had its wings developed
in some remarkable manner in comparison with theother species of the same genus. The rule applies verystrongly in the case of secondary sexual characters, whendisplayed in any unusiial manner. The term, secondary
sexual characters, used by Hunter, applies to characterswhich are attached to one sex, but are not directly
connected with the act of reproduction. The rule applies to males and females ; but as females more rarely
offer remarkable secondary sexual characters, it applies

CHAP. V.

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151

more rarely to them. The rule being so plainly applicable in the case of secondary sexual characters, may be
due to the great variability of these characters, whether
or not displayed in any unusual manner-of which fact
I think there can be little doubt. But that our rule is
not confined to secondary sexual characters is clearlyshown in the case of hermaphrodite cirripedes; andI may here add, that I particularly at.tended to Mr.
Waterhouse’s remark, whilst investigating this Order,and I am fully convinced that the rule almost invariably holds good with cirripedes. I shall, in my future
work, give a list of the more remarkable cases; I will
here only briefly give one, as it illustrates the rule inits largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of theword, very important structures, and they differ extremely little even in different genera; but in theseveral species of one genus, Pyrgoma, these valvespresent a marvellous amount of diversification : thehomologous valves in the different species Being sometimes wholly unlike in shape ; and the amount of variation in the individuals of several of the species is
so great, that it is no exaggeration to state that thevarieties differ more from each other in the charactersof these important valves than do other species of distinct genera.
As birds within the same country vary in a remarkably small degree, I have particularly attended to them,and the rule seems to me certainly to hold good in thisclass. I cannot make out that it applies to plants, andthis would seriously have shaken my belief in its truth,
had not the great variability in plants made it particularly
difficult to compare their relative degrees of variability.
When we see any part or organ developed in aremarkable degree or manner in any species, the fair

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LAWS OF VARIATION.

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presumption is that it is of high importance to thatspecies ; nevertheless the part in this case is eminently
liable to variation. Why should this be so? On theview that each species has been independently created,with all its parts as we now see them, I can see no
explanation. But on the view that groups of species
have descended from other species, and have been niodified through natural selection, I think we can obtainsome light. I n our domestic animals, if any part, or
the whole animal, be neglected and no selection be applied, that part (for instance, the comb in the Dorkingfowl) or the whole breed will cease to have a nearly
uniform character. The breed will then be said to havedegenerated. I n rudimentary organs, and in thosewhich have been but little specialised for any particularpurpose, and perhaps in polymorphic groups, we see anearly parallel natural case ; for in such cases natural
selection either has not or cannot come into full play,
and thus the organisation is left in a fluctuating condition. But what here more especially concerns us is,that in our domestic animals those points, which at thepresent time are undergoing rapid change by continued
selection, are also eminently liable to variation. Looka t the breeds of the pigeon; see what a prodigious
amount of difference there is in the beak of the different tumblers, in the beak and wattle of the different
carriers, in the carriage and tail of our fantails, &c.,these being the points now mainly attended to by English fanciers. Even in the sub-breeds, as in the shortfaced tumbler, it is notoriously difficult to breed themnearly to perfection, and frequently individuals are bornwhich depart widely from the standard. There may betruly said to be a constant struggle going on between,on the one hand, the tendency to reversion to a lessmodified state, as well as an innate tendency to further

CHAP. V.

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153

variability of all kinds, and, on the other hand, thepower of steady selection to keep the breed true. I n
the long run selection gains the day, and we do not
expect to fail so far as to breed a bird as coarse as acommon tumbler from a good short-faced strain. But,
as long as selection is rapidly going on, there may
always be expected to be much variability in the structure undergoing modification. It further deserves
notice that these variable characters, produced by man’s
selection, sometimes become attached, from causes quite
unknown to us, more to one sex than to the other, generally to the male sex, as with the wattle of carriers andthe enlarged crop of pouters.
Now let us turn to nature. When a part has beendeveloped in an extraordinary manner in any one
species, compared with the other species of the same
genus, we may conclude that this part has undergone
an extraordinary amount of modification, since theperiod when the species branched off from the common
progenitor of the genus. This period will seldom beremote in any extreme degree, as species very rarely
endure for more than one geological period. An extraordinary amount of modification implies an unusuallylarge and long-continued amount of variability, which
has continually been accumulated by natural selection
for the benefit of the species. But as the variability ofthe extraordinarily-developed part or organ has been sogreat and long-continued within a period not excessively remote, we might, as a general rule, expect stillto fhd more variability in such parts than in other partsof the organisation, which have remained for a much
longer period nearly constant, And this, I am convinced, is the case. That the struggle between natural
selection on the one hand, and the tendency to reversion and variability on the other hand, will in the

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v.

course of time cease; and that the most abnormallydeveloped organs may be made constant, I can see noreason to doubt. Hence when an organ, however
abnormal it may be, has been t,ransmitted in approximately the same condition to many modified descendants, as in the case of the wing of the bat, it musthave existed, according to my theory, for an immense
period in nearly the same state; and thus it comesto be no more variable than any other structure. It
is only in those cases in which the modification has
been comparatively recent and extraordinarily greatthat we ought to find the generative variability, as it
may be called, still present in a high degree. For inthis case the variability will seldom as yet have been
fixed by the continued selection of the individuals varying in the required manner and degree, and by the continued rejection of those tending to revert to a former
and less modified condition.
The principle included in these remarks may be
extended. It is notorious that specific characters are
more variable than generic. To explain by a simple
example what is meant. If some species in a large genusof plants had blue flowers and some had red, the colour
would be only a specific character, and no one would be
surprised at one of the blue species varying into red, orconversely ; but if all the species had blue flowers, thecolour would become a generic character, and its variation would be a more unusual circumstance. I havechosen this example because an explanation is not inthis case applicable, which most naturalists would advance, namely, that specific characters are more variable
than generic, because they are taken from parts of lessphysiological importance than those commonly used forclassing genera. I believe this explanation is partly,
yet only indirectly, true; I shall, however, have to re-CHAP. Y.

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turn to this subject in our chapter on Classification. It
would be almost superfluous to adduce evidence insubport of the above statement, that specific characters
are more variable than generic ; but I have repea.tedly
noticed in works on natural history, that when an author
has remarked with surprise that some important organor part, which is generally very constant throughout
large groups of species, has dzfered considerably inclosely-allied species, that it has, also, been variable in
the individuals of some of the species. And this factshows that a character, which is generally of genericvalue, when it sinks in value and becomes only of specific value, often becomes variable, though its physiological importance may remain the same. Something of
the same kind applies to monstrosities : at least Is. Geoffroy St. Hilaire seems to entertain no doubt, that themore an organ normally differs in the different speciesof the same group, the more subject it is to individual
anomalies.
On the ordinary view of each species having been
independently created, why should that part of thestructure, which differs from the same part in otherindependently-created species of the same genus, be morevariable than those parts which are closely alike in theseveral species? I do not see that any explanation
can be given. But on the view of species being only
strongly marked and fixed varieties, we might surely
expect to find them still often cont,inuing to vary in thoseparts of their structure which have varied within a moderately recent period, and which have thus come to differ.
Or to state the case in another manner:-the points inwhich all the species of a genus resemble each other,and in which they differ from the species of some othergenus, are called generic characters ; and these characters in common I attribute to inheritance from a common

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v.

progenitor, for it can rarely have happened that natural
selection will have modified several species, fitted tomore or less widely-different habits, in exactly the samemanner : and as these so-called generic characters havebeen inherited from a remote period, since that period
when the species fist branched off from their common
progenitor, and subsequently have not varied or come todiffer in any degree, or only in a slight degree, it is notprobable that they should vary at the present day. On
the other hand, the points in which species differ fromother species of the same genus, are called specific characters; and as these specific characters have varied
and come to differ within the period of the branching
off of the species from a common progenitor, it is probable that they should still often be in some degree
variable,-at least more variable than those parts of
the organisation which have for a very long period
remained constant.
I n connexion with the present subject, I will makeonly two other remarks. I think it will be admitted,
without my entering on details, that, secondary sexual
characters are very variable; I think it also will be
admitted that species of the same group differ fromeach other more widely in their secondary sexual characters, than in other parts of their organisation ; compare, for instance, the amount of difference between themales of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount ofdifference between their females; and the truth of this
proposition will be granted. The cause of the originalvariability of secondary sexual characters is not manifest; but we can see why these characters should nothave been rendered as constant and uniform as otherparts of the organisation ; for secondary sexual characters have been accumulated by sexual selection, which

CEAP. V.

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is less rigid in its action than ordinary selection, as it
does not entail death, but only gives fewer offspring to
the-less favoured males. Whatever the cause may beof the variability of secondary sexual characters, as they
are highly variable, sexual selection will have had awide scope for action, and may thus readily have succeeded in giving to the species of the same group agreater amount of difference in their sexual characters,
than in other parts of their structure.
It is a remarkable fact, that the secondary sexual
differences between the two sexes of the same species
are generally displayed in the very same parts of theorganisation in which the different species of the same
genus differ from each other. Of this fact I will givein illustration two instances, the first which happen
to stand on my list; and as the differences in thesecases are of a very unusual nature, the relation canhardly be accident,al. The same number of joints
in the tarsi is a character generally common to verylarge groups of beetles, but in the Engidae, as Westaood
has remarked, the number varies greatly; and thenumber likewise differs in the two sexes of the same
species: again in fossorial hymenoptera, the manner ofneuration of the wings is a character of the highest
importance, because common to large groups ; but incertain genera the neuration differs in the different species, and likewise in the two sexes of the same species.
This relation has a clear meaning on my view of thesubject : I look at all the species of the same genus ashaving as certainly descended from the same progenitor,
as have the two sexes of any one of the species. Consequently, whatever part of the structure of the common
progenitor, or of its early descendants, became variable ;variations of this part would, it is highly probable, betaken advantage of by natural and sexual selection, in

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order to fit the several species to their several placesin the economy of nature, and likewise to fit the twosexes of the same species to each other, or to fit themales and females to different habits of life, or the
males to struggle with other males for the possessionof the females.
Finally, then, I conclude that the greater variabilityof specific characters, or those which distinguish species
from species, than of generic characters, or those which
the species possess in common ;-that the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with
Ithe same part in its congeners ; and the not great'degree of variability in a part, however extraordinarily'it may be developed, if it be common to a whole groupof species ;-that the great variability of secondary
sexual characters, and the great amount of difference inthese same characters between closely allied species ;that secondary sexual and ordinary specific differences
are generally displayed in the same parts of the organisation,-are all principles closely connected together.
All being mainly due to the species of the same grouphaving descended from a common progenitor, fromwhom they have inherited much in common,-to parts
which have recently and largely varied being more
likely still to go on varying than parts which have long
been inherited and have not varied,-to natural selection having more or less completely, according to thelapse of time, overmastered the tendency to reversion
and to further variability,-to sexual selection being
less rigid than ordinary selection,-and to variations inthe same parts having been accumulated by naturaland sexual selection, and thus adapted for secondary
sexual, and for ordinary spec& purposes.

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159

Distinct species present analogous variations ; and avariety of one species often assumes some of the charactersof an allied species, or reverts to some o f the characters ofan ear& pogenitor.-These
propositions will be most
readily understood by looking to our domestic races.
The most distinct breeds of pigeons, in countries mostwidely apart, present subvarieties with reversed featherson the head and feathers on the feet,-characters notpossessed by the aboriginal rock-pigeon ; these then
are analogous variations in two or more distinct races.
The frequent presence of fourteen or even sixteen tailfeathers in the pouter, may be considered as a variation
representing the normal structure of another race, thefantail. I presume that no one will doubt that all suchanalogous variations are due to the several races ofthe pigeon having inherited from a common parent thesame constitution and tendency to variation, when act,edon by similar unknown influences. I n the vegetable
kingdom we have a case of analogous variation, in theenlarged stems, or roots as commonly called, of theSwedish turnip and Ruta baga, plants which several
botanists rank as varieties produced by cultivation froma common parent : if this be not so, the case will thenbe one of analogous variation in two so-called distinct
species; and to these a third may be added, namely,
the common turnip. According to the ordinary viewof each species hat-ing been independently created,
we should have to attribute this similarity in the enlarged stems of these three plants, not to the Vera causa
of community of descent, and a consequent tendency
to vary in a like manner, but to three separate yetclosely related acts of creation.
With pigeons, however, we have another case, namely,
the occasional appearance in all the breeds, of slatyblue birds with two black bars on the wings, a white

160

LAWS OF VARIATION.

CHAP. v.

rump, a bar at the end of the tail, with the outerfeathers externally edged near their bases with white.
As all these marks are characteristic of the parent rockpigeon, I presume that no one will doubt that this isa case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may I
think confidently come to this conclusion, because, as
we have seen, these coloured marks are eminently liable
to appear in the crossed offspring of two distinct anddifferently coloured breeds; and in this case there isnothing in the external conditions of life to cause thereappearance of the slaty-blue, with the several marks,
beyond the influence of the mere act of crossing on thelaws of inheritance.
No doubt it is a very surprising fact that charactersshould reappear after having been lost for many, perhaps
for hundreds of generations. But when a breed has
been crossed only once by some other breed, the offspringoccasionally show a tendency to revert in character tothe foreign breed for many generations-some say, fora dozen or even a score of generations. After twelve
generations, the proportion of blood, to use a common
expression, of any one ancestor, is only 1 in 2018 ; andyet, as we see, it is generally believed that a tendency
to reversion is retained by this very small proportion offoreign blood. I n a breed which has not been crossed,
but in which 60th parents have lost some character
which their progenitor possessed, the tendency, whether
strong or weak, t o reproduce the lost character mightbe, as was formerly remarked, for all that we can see
to the contrary, transmitted for almost any number ofgenerations. When a character which has been lost ina breed, reappears after a great number of generations,the most probable hypothesis is, not that the offspring suddenly takes after an ancestor some hundred generations

CHAP.V.

LAWS OF VARIATION.

161

distant, but that in each successive generation there has
been a tendency to reproduce the character in question,
which at last, under unknown favourable conditions,gains an ascendancy. For instance, it is probable thatin each generation of the barb-pigeon, which produces
most rarely a blue and black-barred bird, there has
been a tendency in each generation in the plumage toassume this colour. This view is hypothetical, but could
be supported by some facts; and I can see no more
abstract improbability in a tendency to produce any character being inherited for an endless number of generations, than in quite useless or rudimentary organs being,
as we all know them to be, thus inherited. Indeed, we
may sometimes observe a mere tendency to produce a
rudiment inherited : for instance, in the common snapdragon (Antirrhinum) a rudiment of a fifth stamen so
often appears, that this plant must have an inheritedtendency to produce it.
As all the species of the same genus are supposed,on my theory, to have descended from a common parent,
it might be expected that they would occasionally varyin an analogous manner ; so that a variety of one species
would resemble in some of its characters another species ;this other species being on my view only a well-marked
and permanent variety. But characters thus gained
would probably be of an unimportant nature, for thepresence of all important characters will be governedby natural selection, in accordance with the diverse
habits of the species, and will not be left to the mutual
action of the conditions of life and of a similar inherited constitution. It might further be expectedthat the species of the same genus would occasionallyexhibit reversions to lost ancestral characters. As, however, we never know the exact character of the common
ancestor of a group, we could not distinguish these two

162

LAWS OF VARIATION.

CHAP.

v.

cases: if, for instance, we did not know that the rockpigeon was not feather-footed or turn-crowned, we could
not have told, whether these characters in our domestic
breeds were reversions or only analogous variations ; but
we might have inferred that the blueness was a case ofreversion, from the number of the markings, which arecorrelated with the blue tint, and which it does not appear probable would all appear together from simple
variation. More especially we might have inferred this,from the blue colour and marks so often appearing
when distinct breeds of diverse colours are crossed.
Hence, though under nature it must generally be left
doubtful, what cases are reversions to an anciently existing character, and what are new but analogous variations, pet we ought, on my theory, sometimes to findthe varying offspring of a species assuming characters(either from reversion or from analogous variation)
which already occur in some other members of thesame group. And this undoubtedly is the case innature.
A considerable part of the difficulty in recognising avariable species in our systematic works, is due to its
varieties mocking, as it were, some of the other species of the same genus. A considerable catalogue, also,
could be given of forms intermediate between two otherforms, which themselves must be doubtfully ranked aseither varieties or species; and this shows, unless allthese forms be considered as independently createdspecies, that the one in varying has assumed some of
the characters of the other, so as to produce the intermediate form. But the best evidence is afforded byparts or organs of an important and uniform natureoccasionally varying so as to acquire, in some degree,
the character of the same part or organ in an allied
species. I have collected a long list of such cases ; but

CHAP. V.

LAWS OF VARIATION.

163

here, as before, 1 lie under a great disadvantage in
not being able to give them. I can only repeat thatsuch cases certainly do occur, and seem to me very
remarkable.
I will, however, give one curious and complex case,not indeed as affecting any important character, butfrom occurring in several species of the same genus,
partly under domestication and partly under nature.
It is a case apparently of reversion. The ass not rarely
has very distinct transverse bars on its legs, like thoseon the legs of the zebra: it has been asserted that theseare plainest in the foal, and from inquiries which I have
made, I believe this to be true. It has also been asserted that the stripe on each shoulder is sometimes
double. The shoulder-stripe is certainly very variable
in Iength and outline. A white ass, but not an albino,
has been described without, either spinal or shoulder
stripe ; aud these stripes are sometimes very obscure, or
actually quite lost, in dark-coloured asses. The koulanof Pallas is said to have been seen with a double shoulderstripe. The hemionus has no shoulder-stripe ; but traces
of it, as stated by Mr. Blyth and others, occasionallyappear: and I have been informed by Colonel Poolethat the foals of this species are generally striped on
the legs, and faintly on the shoulder. The quagga,
though so plainly barred like a zebra over the body, is
without bars on the legs; but Dr. Gray has figured
one specimen with very distinct zebra-like bars on thehocks.
With respect to the horse, I have collected cases inEngland of the spinal stripe in horses of the most distinct breeds, and of all colours ; transverse bars on thelegs are not rare in duns, mouse-duns, and in one
instance in a chestnut: a faint shoulder-stripe may
sometimes be seen in duns, and I have seen a trace in a

164

I

LAWS OF VARIATION.

CHAP.V.

bay horse. My son made a careful examination andsketch for me of a dun Belgian cart-horse with a double
stripe on each shoulder and with leg-stripes ; and a man,whom I can implicitly trust, has examined for me asmall dun Welch pony with three short parallel stripes
on each shoulder.
In the north-west part of India the Kattywar breedof horses is so generally striped, that, as I hear from
Colonel Poole, who examined the breed for the Indian
Government, a horse without stripes is not considered as
purely-bred. The spine is always striped ; the legs are
generally barred ; and the shoulder-stripe, which is sometimes double and sometimes treble, is common ; the sideof the face, moreover, is sometimes striped. The stripes
are plainest in the foal ; and sometimes quite disappear
in old horses. Colonel Poole has seen both gray andbay Kattywar horses striped when first foaled. I
have, also, reason to suspect, from information given
me by M i . W. W. Edwards, that with the English racehorse the spinal stripe is much commoner in the foal
than in the full-grown animal. Without here entering
on further details, I may state that I have collected casesof leg and shoulder stripes in horses of very different
breeds, in various countries from Britain to Eastern
China; and from Norway in the north to the Malay
Archipelago in the south. In all parts of the worldthese stripes occur far oftenest in duns and mouse-duns ;by the term dun a large range of colour is included,from one between brown and black to a close approachto cream-colour.
I am aware that Colonel Hamilton Smith, who has
written on this subject, believes that the several breedsof the horse have descended from several aboriginal
species-one of which, the dun, was striped; and thatthe above-described appearances are all due to ancientCHAP.

\’.

LAWS OF VARIATION.

165

crosses with the dun stock. But I am not at all satisfied
with this theory, and should be loth to apply it to breeds
so distinct as the heavy Belgian cart-horse, Welch ponies,
cobs, the lanky Kattywar race, &c., inhabiting the most
distant parts of the world.
Now let us turn to the effects of crossing the several
species of the horse-genus. Rollin asserts, that thecommon mule from the ass and horse is particularly apt
to have bars on its legs. I once saw a mule with
its legs so much striped that any one at first would
have thought that it must have been the product of a,zebra ; and Mr. W.C. Martin, in his excellent treatiseon the horse, has given a figure of a similar mule. In
four coloured drawings, which I have seen, of hybrik
between the ass and zebra, the legs were much more
plainly barred than the rest of the body; and in oneof them there was a double shoulder-stripe. I n Lord
Moreton’s famous hybrid from a chestnut mare and malequagga, the hybrid, and even the pure offspring subsequently produced from the mare by a black Arabiansire, were much more plainly barred across the legs thanis even the pure quagga. Lastly, and this is anothermost remarkable case, a hybrid has been figured by
Dr. Gray (and he informs me that he knows of a secondcase) from the ass and the hemionus ; and this hybrid,
though the ass seldom has stripes on its legs and thehemionus has none and has not even a shoulder-stripe,
nevertheless had all four legs barred, and had three
short shoulder-stripes, like those on the dun Welch
pony, and even had some zebra-like stripes on thesides of its face. With respect to this last fact, I
was so convinced that not even a stripe of colour
appears from what would commonly be called an accident, that I was led solely from the occurrence of theface-stripes on this hybrid from the ass and hemionus,

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LAWS OF VARIATION.

CHAP. V.

to ask Colonel Poole whether such face-stripes everoccur in the eminently striped Kattywar breed of horses,
and was, as we have seen, answered in the affirmative.
What now are we to say to these several facts? We
see several very distinct species of the horse-genus
becoming, by simple variation, striped on the legs likea zebra, or striped on the shoulders like an ass. I n thehorse we see this tendency strong whenever, a dun tint
appears-a tint which approaches to that of the generalcolouring of t'he other species of the genus. The
appearance of the stripes is not accompanied by any
change of form or by any other new character. We see
this tendency to become striped most strongly displayedin hybrids from between several of the most distinct
species. Now observe the case of the several breeds of
pigeons : they are descended from a pigeon (including
two or three sub-species or geographical races) of a
bluish colour, with certain bars and other marks ; andwhen any breed assumes by simple variation a bluishtint, these bars and other marks invariably reappear ;but without any other change of form or character.
When the oldest and truest breeds of various colours
are crossed, we see a strong tendency for the blue tint
and bars and marks to reappear in the mongrels. I
have stated that the most probable hypothesis to accountfor the reappearance of very ancient characters, isthat there is a tendency in the young of each successive
generation to produce the long-lost character, and that
this tendency, from unknown causes, sometimes prevails.
And we have just seen that in several species of thehorse-genus the stripes are either plainer or appear
more commonly in the young than in the old. Callthe breeds of pigeons, some of which have bred true
for centuries, species; and how exactly parallel isthe case with that of the species of the horse-genus!

CHAP. V.

SUMMARY.

167

For myself, I venture confidently to look back thousandson thousands of generations, and I see an animal striped
like a zebra, but perhaps otherwise very differently
constructed, the common parent of our domestic horse,
whether or not it be descended from one or more wild
stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created, will, I presume, assert that eachspecies has been created with a tendency to vary, bothunder nature and under domestication, in this particular manner, so as often to become striped likeother species of the genus; and that each has beencreated with a strong tendency, when crossed with species
inhabiting distant quarters of the world, to produce
hybrids resembling in their stripes, not their own
parents, but other species of the genus. To admit
this view is, as it seems to me, to reject a real for an
unreal, or at least for an unknown, cause. It makes
the works of God a mere mockery and deception; I
would almost as soon believe with the old and ignorantcosmogonists, that fossil shells had never lived, but had
been created in stone so as to mock the shells now
living on the sea-shore.
Summary.-Our ignorance of the laws of variation ie
profound. Not in one w e out of a hundred can we
pretend to a s s i p any reason why this or that partdiffers, more or less, from the same part in the parents.
But whenever we have the means of instituting a comparison, the same laws appear to have acted in producing the lesser differences between varieties of thesame species, and the greater differences between speciesof the same genus. The external conditions of life, as
climate and food, &c., seem to have induced some slight
modifications. Habit in producing constitutional dif-

168

LAWS OF VARIATION.

CHAP.

v.

ferences, and use in strengthening, and disuse in weakening and diminishing organs, seem to have been morepotent in their effects. Homologous parts tend to varyin the same way, and homologous parts tend to cohere.
Modifications in hard parts and in external parts sometimes affect softer and internal parts. When one partis largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of thestructure which can be saved without detriment to theindividual, will be saved. Changes of structure at an
early age will generally affect parts subsequently developed ; and there are very many other correlations ofgrowth, the nature of which we are utterly unable tounderstand. Multiple parts are variable in number andin structure, perhaps arising from such parts not having
been closely specialised to any particular function, sothat their modifications have not been closely checkedby natural selection. It is probably from this samecause that organic beings low in the scde of nature are
more variable than those which have their whole organisation more specialised, and are higher in the scale.
Rudimentary organs, from being useless, will be disregarded by natural selection, and hence probably arevariable. Spec& characters-that is, the characterswhich have come to differ since the several species ofthe same genus branched off from a common parentare more variable than generic characters, or thosewhich have long been inherited, and have not differedwithin this same period. I n these remarks we havereferred to special parts or organs being still variable,
because they have recently varied and thus come todiffer; but we have also seen in the second Chapterthat the same principle applies to the whole individual ;for in a district where many species of any genus arefound-that is, where there has been much former

CKAP.V,

SUMMARY.

169

variation and differentiation,or where the manufactoryof new specific forms has been actively at work-there,
on an average, we now find most varieties or incipient
species. Secondary sexual characters are highly variable, and such characters differ much in the speciesof the same group. Variability in the same parts of
the organisation has generally been taken advantage
of in giving secondary sexual differences to the sexesof the same species, and specific differences to theseveral species of the same genus. Any part or organ
developed to an extraordinary size or in an extraordinary manner, in comparison with the same part ororgan in the allied species, must have gone through an
extraordinary amount of modification since the genus
arose ; and thus we can understand why it should often
still be variable in a much higher degree than otherparts ; for variation is a long-continued and slow process, and natural selection will in such cases not as
yet have had time to overcome the tendency to further
variability and to reversion to a less modified state. But
when a species with any extraordinarily-developed organ
has become the parent of many modified descendants-which on my view must be a very slow process,requiring a long lapse of time-in this case, natural
selection may readily have succeeded in giving a fixedcharacter to the organ, in however extraordinary amanner it may be developed. Species inheriting nearly
the same constitution from a common parent and exposed to similar influences will naturally tend to present
analogous variations, and these same species may occasionally revert to some of the characters of their ancient
progenitors. Although new and important modifications may not arise from reversion and analogous variation, such modifications will add to the beautiful andharmonious diversity of nature.

170

LAWS OF VARIATION.

CHAP. V.

Whatever the cause may be of each slight differencein the offspring from their parents-and a cause foreach must e x i s L i t is the steady accumulation, throughnatural selection, of such differences, when beneficial tothe individual, that gives rise to all the more important
modifications of structure, by which the innumerable
beings on the face of this earth are enabled to struggle
with each other, and the best adapted to survive.

CHAP. VI.

DIFFICULTIES ON THEORY.

CHAPTER

171

vr.

DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with modification-TransitionsAbsence or rarity of transitional varieties-Transitions in habitsof life-Diversified habits in the same species-Species with
habits widely different from those of their allies-Organs of
extreme perfection-Means of transition-Cases of difficultyNatura non facit saltum-Organs of small importance-Organs
not in all cases absolutely perfect-The law of Unity of Typeand of the Conditions of Existence embraced by the theory ofKatural Selection.

LONGbefore having arrived at this part of my work, a
crowd of difficulties will have occurred to the reader.
Some of them are so grave that to this day I can neverreflect on them without being staggered ; but, to the bestof my judgment, the greater number are only apparent,
and those that are real are not, I think, fatal to mytheory.
These difficulties and objections may be classed under
the following heads :-Firstly,
why, if species havedescended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional
forms? Why is not all nature in confusion instead of
the species being, as we see them, well defined?
Secondly, is it possible that an animal having, forinstance, the structure and habits of a bat, could havebeen formed by the modification of some animal withwholly different habits? Can we believe that natural
selection could produce, on the one hand, organs oftrifling importance, such as the tail of a giraffe, wbich
serves as a fly-flapper, and, on the other hand, organs of

172

DIFFICULTIES ON THEORY.

CHAP. VI.

such wonderful structure, as the eye, of which we hardly
as yet fully understand the inimitable perfection ?
Thirdly, can instincts be acquired and modified throughnatural selection? What shall we say to so marvellous
an instinct as that which leads the bee to make cells,
which have practically anticipated t,he discoveries of
profound mathematicians ?
Fourthly, how can we account for species, when crossed,
being sterile and producing sterile offspring, whereas,
when varieties are crossed, their fertility is unimpaired ?
The two first heads shall be here discussed-Instinct
and Hybridism in separate chapters.
On the absence or rarity of transitional varieties.As natural selection acts solely by the preservation of
profitable modifications, each new form will tend in a
fully-stocked country to take the place of, and finally to
exterminate, its own less improved parent or other lessfavoured forms with which it comes into Competition.
Thus extinction and natural selection will, as we haveseen, go hand in hand. Hence, if we look at each species
as descended from some other unknown form, both theparent and all the transitional varieties will generally
have been exterminated by the very process of formation and perfection of the new form.
But, as by this theory innumerable transitional forms
must have existed, why do we not find them embedded
in countless numbers in the crust of the earth? It will
be much more convenient to discuss this question in thechapter on the Imperfection of the geological record ;and I will here only state that I believe the answer
mainly lies in the record being incomparably less perfectthan is generally supposed; the imperfection of therecord being chiefly due to organic beings not inhabiting

CHAP. VI.

TRANSITIONAL VARIETIES.

173

profound depths of the sea, and to their remains being
embedded and preserved to a future age only in massesof sediment sufficiently thick and extensive to withstandan enormous amount of future degradation; and suchfossiliferous masses can be accumulated only where muchsediment is deposited on the shallow bed of the sea,
whilst it slowly subsides. These contingencies willconcur only rarely, and after enormously long intervals.
Whilst the bed of the sea is stationary or is rising, or
when very little sediment is being deposited, there willbe blanks in our geological history. The crust of theearth is a vast museum; but the natural collections
have been made only at intervals of time immensely
remote.
But it may be urged that when several closely-allied
species inhabit the same territory we surely ought tofind at the present time many transitional forms. Let
us take a simple case : in travelling from north to south
over a continent, we generally meet at successive intervals with closely allied or representative species, evidently slling nearly the same place in the natural
economy of the land. These representative species
often meet and interlock; and as the one becomes
rarer and rarer, the other becomes more and more frequent, till the one replaces the other. But if we compare these species where they intermingle, they are
generally as absolutely distinct from each other in every
detail of structure as are specimens taken from themetropolis inhabited by each. By my theory these
allied species have descended from a common parent;and during the process of modification, each has become
adapted to the conditions of life of its own region, andhas supplanted and exterminated its original parent
and all the transitional varieties between its past andpresent states. Hence we ought not to expect at the

174

DIFFICULTIES ON THEORY.

CHAP. VI.

present time to meet with numerous transitional varieties in each region, though they must have existed
there, and may be embedded there in a fossil condition.
But in the intermediate region, having intermediate
conditions of life, why do we not now find closely-linking
intermediate varieties? This difficulty for a long timequite confounded me. But I think it can be in largepart explained.
I n the first place we should be extremely cautious
in inferring, because an area is now continuous, that it
has been continuous during a long period. Geology
would lead us to believe that almost every continent
has been broken up into islands even during the later
tertiary periods ; and in such islands distinct species
might have been separately formed without the possibility of intermediate varieties existing in the intermediate zones. By changes in the form of the land andof climate, marine areas now continuous must often
have existed within recent times in a far less continuous
and uniform condition than at present. But I will passover this way of escaping from the difficulty; for I
believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not
doubt that the formerly broken condition of areas now
continuous has played an important part in the formation of new species, more especially with freely-crossing
and wandering animals.
I n looking at species as they are now distributed
over a wide area, we generally find them tolerably
numerous over a large territory, then becoming somewhat abruptly rarer and rarer on the confines, andfinally disappearing. Hence the neutral territory between two representative species is generally narrow incomparison with the territory proper to each. We see
the same fact in ascending mountains, and sometimes

CHAP. VI.

TRANSITIONAL VARIETIES.

175

it is quite remarkable how abruptly, as Alph. De Candolle has observed, a common alpine species disappears. The same fact has been noticed by Forbes insounding the depths of the sea with1 the dredge. To
those who look at climate and the physical conditions oflife as the all-important elements of distribution, thesefacts ought to cause surprise, as climate and height or
depth graduate away insensibly. But when we bear inmind that almost every species, even in its metropolis,
would increase immensely in numbers, were it not forother competing species ; that nearly all either prey onor serve as prey for others ; in short, that each organic
being is either directly or indirectly related in the mostimportant manner to other organic beings, we must seethat the range of the inhabitants of any country by
no means exclusively depends on insensibly changing
physical conditions, but in large part on the presence ofother species, on which it depends, or by which it isdestroyed, or with which it comes into competition ; andas these species are already defined objects (however
they may have become so), not blending one into anotherby insensible gradations, the range of any one species,
depending as it does on the range of others, will tend tobe sharply defined. Moreover, each species on the confines of its range, where it exists in lessened numbers,
will, during fluctuations in the number of its enemies
or of its prey, or in the seasons, be extremely liable
to utter extermination ; and thus its geographical range
will come to be still more sharply defined.
If I am right in believing that allied or representative
species, when inhabiting a continuous area, are generally so distributed that each has a wide range, witha comparatively narrow neutral territory between them,in which they become rather suddenly rarer and rarer ;then, as varieties do not essentially differ from species,

176

DIFFICULTIES ON THEORY.

CHAP.VI.

the same rule will probably apply to both ; and if we inimagination adapt a varying species to a very large area,
we shall have to adapt two varieties to two large areas,
and a third variety to a narrow intermediate zone. Theintermediate variety, consequently, will exist in lesser
numbers from inhabiting a narrow and lesser area ; andpractically, as far as I can make out, this rule holds goodwith varieties in a state of nature. I have met withstriking instances of the rule in the case of varieties
intermediate between well-marked varieties in the genus
Balanus. And it would appear from information given
me by Mr. Watson, Dr. &a Gray, and Mr. Wollaston,that generally when varieties intermediate between
two other forms occur, they are much rarer numericallythan the forms which they connect. Now, if we maytrust these facts and inferences, and therefore conclude
that varieties linking two other varieties together havegenerally existed in lesser numbers than the forms which
they connect, then, I think, we can understand why intermediate varieties should not endure for very long
periods;-why as a general rule they should be exterminated and disappear, sooner than the forms which
they originally linked 1ogether
For any form existing in lesser numbers would, as
already remarked, run a greater chance of being exterminated than one existing in large numbers; and in
this particular case the intermediate form would beeminently liable to the inroads of closely allied forms
existing on both sides of it. But a far more importantconsideration, as I believe, is that, during the processof further modification, by which two varieties are supposed on my theory to be converted and perfected into
two distinct species, the two which exist in larger
numbers from inhabiting Lrger areas, will have a great
advantage over the intermediate variety, which exists

CHAP. VI.

TRANSITIONAL VARIETIES.

177

in smaller numbers in a narrow and intermediate zone.
For forms existing in larger numbers will always havea Letter chance, within any given period, of presenting
further favourable variations for natural selection toseize on, than will the rarer forms which exist in lesser
numbers. Hence, the more common forms, in the race
for life, will tend to beat and supplant the less common
forms, for these will be more slowly modified and improved. It is the same principle which, as I believe,
accounts for the common species in each country, as
shown in the second chapter, presenting on an averagea greater number of well-marked varieties than do therarer species. I may illustrate what I mean by s u p
posing three varieties of sheep to be kept, one adapted
to an extensive mountainous region ; a second to a comparatively narrow, hilly tract; and a third to wide
plains at the base; and that the inhabit,ants are alltrying with equal steadiness and skill to improve theirstocks by selection; the chances in this case will bestrongly in favour of the great holders on the mountains or on the plains improving their breeds more
quickly than the small holders on the intermediatena.rrow, hilly tract ; and consequently the improved
mountain or plain breed will soon take the place ofthe less improved hill breed ; and thus the two breeds,
which originally existed in greater numbers, will comeinto close contact with each other, without the interposition of the supplanted, intermediate hill-variety.
To sum up, I believe that species come to be tolerably
well-defined objects, and do not at any one period present an inextricable chaos of varying and intermediate
links: firstly, because new varieties are very slowly
formed, for variation is a very slow process, andnatural selection can do nothing until favourable variations chance to occur, and until a place in the natural

178

DIFFICULTIES ON THEORP.

CHAP. VI.

polity of the country can be better filled by somemodification of some one or more of its inhabitants.
And such new places will depend on slow changes ofclimate, or on the occasional immigration of new inhabitants, and, probably, in a still more important degree,
on some of the old inhabitants becoming slowly modified, with the new forms thus produced and the oldones acting and reacting on each other. So that, in
any one region and at any one time, we ought only to
see a few species presenting slight modifications of structure in some degree permanent; and this assuredly we
do see.
Secondly, areas now continuous must often haveexisted within the recent period in isolated portions, inwhich many forms, more especially amongst the classes
which unite for each birth and wander much, may haveseparately been rendered sufficiently distinct to rank as
representative species. I n this case, intermediate varieties between the several representative species andtheir common parent, must formerly have existfed ineach broken portion of the land, but these links will
have been supplanted and exterminated during the process of natural selection, so that they will no longer
exist in a living state.
Thirdly, when two or more varieties have been formed
in different portions of a strictly continuous area, intermediate varieties will, it is probable, at first have been
formed in the intermediate zones, but they will generally have had a short duration. For these intermediate varieties will, from reasons already assigned(namely from what we know of the actual distributionof closely allied or representative species, and likewiseof acknowledged varieties), exist in the intermediatezones in lesser numbers than the varieties which theytend to connect. From this cause alone the interme-

CIIAP. VI.

TRANSITIONAL HABITS.

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diate varieties will be liable to accidental extermination ;and during the process of further modification throughnatural selection, they will almost certainly be beaten
and supplanted by the forms which they connect; forthese from existing in greater numbers will, in theaggregate, present more variation, and thus be further
improved through natural selection and gain further
advantages.
Lastly, looking not to any one time, but to all time,
if my theory be true, numberless intermediate varieties,
linking most closely all the species of the game group
together, must assuredly have existed; but the very
process of natural selection constantly tends, as has been
so often remarked, to exterminate the parent-forms andt,he intermediate links. Consequently evidence of theirformer existence could be found only amongst fossil
remains, which are preserved, as we shall in a future
chapter attempt to show, in an extremely imperfect andintermittent record.

On the origin and transitions of organic beings withpeculiar habits and atructure.-It has been asked by
the opponents of such views as I hold, how, for instance,a land carnivorous animal could have been converted
into one with aquatic habit,s; for how could the animal
in its transitional state have subsisted? It would beeasy to show that within the same group carnivorous
animals exist having every intermediate grade between
truly aquatic and strictly terrestrial habits; and as
each exists by a struggle for life, it is clear that each iswell adapted in its habits to its place in nature. Lookat the Mustela vison of North America, which hm
webbed feet and which resembles an otter in its fur,short legs, and form of tail ; during summer this animal
dives for and preys on fish, but during the long winter

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DIFFICULTIES ON THEORY.

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it leaves the frozen waters, and preys like other polecats on mice and land animals. If a different case had
been taken, and it had been asked how an insectivorous
quadruped could possibly have been converted into 5t
flying bat, the question would have been far more difficult, and I could have given no answer. Yet I thinksuch difficulties have very little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out of the many striking cases which I
have collected, I can give only one or two instancesof transitional habits and structures in closely allied
species of the same genus; and of diversified habits,
either constant or occasional, in the same species. And
it seems to me that nothing less than a long list of suchcases is sufficient to lessen the difficulty in any pasticular case like that of the bat.
Look at the family of squirrels; here we have thefinest gradation from animals with their tails only
slightly flattened, and from others, as Sir J. Richardson
has remarked, with the posterior part of their bodies
rather wide and with the skin on their flanks rather full,to the so-called flying squirrels; and flying squirrels
have their limbs and even the base of the tail united bya broad expanse of skin, which serves as a parachuteand allows them to glide through the air to an astonishing distance from tree to tree. We cannot doubtthat each structure is of m e to each kind of squirrel in
its own country, by enabling it to escape birds or beastsof prey, or to collect food more quickly, OT, as there
is reason to believe, by lessening the danger from occasional falls. But it does not follow from this fact thatthe structure of each squirrel is the best that it is possible to conceive under all natural conditions. Let theclimate and vegetation change, let other competingrodents or new beasts of prey immigrate, or old ones

CHAP.VI.

TRANSITIOK4L HABITS.

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become modified, and all analogy would lead us tobelieve that some at least of the squirrels would decreasein numbers or become exterminated, unless they alsu
became modified and improved in structure in a corresponding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, inthe continued preservation of individuals with fuller andfuller flank-membranes, each modification being usefd,
each being propagated, until by the accumulated effectsof this process of natural selection, a perfect so-called
flying squirrel was produced.
Now look at the Galeopithecus or flying lemur,
which formerly was falsely ranked amongst bats. It
has an extremely wide flank-membrane, stretching fromthe corners of the jaw to the tail, and including thelimbs and the elongated h g e r s : the flank-membrane
is, also, furnished with an extensor muscle. Although
no graduated links of structure, fitted for gliding throughthe air, now connect the Galeopithecus with the other
Lemuridae, yet I can see no dBcidty in supposing thatsuch links formerly existed, and that each had been
formed by the same steps as in the case of the less perfectly gliding squirrels ; and that each grade of structure
had been useful to its possessor. Nor can I see any insuperable difficulty in further believing it possible thatthe membrane-connected fingers and fore-arm of the
Galeopithecus might be greatly lengthened by natural
selection; and this, as far as the organs of flight areconcerned, would convert it into a bat. I n bats whichhave the wing-membrane extended from the top of theshoulder to the tail, including the hind-legs, we perhaps see traces of an apparatus originally constructed
for gliding through the air rather than for flight.
If about a dozen genera of birds had become extinct
or were unknown, who would have ventured to have

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DIFFICULTIES ON THEORY.

CHAP,VJ.

surmised that birds might have existed which used theirwings solely as flappers, like the logger-headed duck(Micropterus of Eyton) ; as fins in the water and front
legs on the land, like the penguin; as sails, like theostrich; and functionally for no purpose, like theApteryx. Yet the structure of each of these birds isgood for it, under the conditions of life to which it is
exposed, for each has to live by a struggle; but it is
not necessarily the best possible under all possible conditions. It must not be inferred from these remarksthat any of the grades of wing-structure here alluded to,which perhaps may all have resulted from disuse, indicate the natural steps by which birds have acquired
their perfect power of flight ; but they serve, at least,
to show what diversified means of transition are possible.
Seeing that a few members of such water-breathingclasses as the Crustacea and Mollusca are adapted tolive on the land, and seeing that we hai-e flying birds
and mammals, flying insects of the most diversified
types, and formerly had flying reptiles, it is conceivablethat flying-fish, which now glide far through the air,slightly rising and turning by the aid of their flutteringfins, might have been modified into perfectly winged
animals. If this had been effected, who would haveever imagined that in an early transitional state they
had been inhabitants of the open ocean, and had usedtheir incipient organs of flight exclusively, as far as weknow, to escape being devoured by other fish?
When we see any structure highly perfected for any
particular habit, as the wings of a bird for flight, weshould bear in mind that animals displaying early transitional grades of the structure will seldom continue toexist to the present day, for they will have been supplanted by the very process of perfection through natural
selection. Furthermore, we may conclude that transi-

CHAP. VI.

TRANSITIONAL HABITS.

183

tional grades between structures fitted for very different
habits of life will rarely have been developed a t an early
period in great numbers and under many subordinate
forms. Thus, to return to our imaginary illustration of
the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed undermany subordinate forms, for taking prey of many kinds
in many ways, on the land and in the water, until theirorgans of flight had come to a high stage of perfection,
so as to have given them a decided advantage overother animals in the battle for life. Hence the chanceof discovering species with transitional grades of structure in a fossil condition will always be less, from theirhaving existed in lesser numbers, than in the case of
species with fully developed structures.
I will now give two or three instances of diversified
and of changed habits in the individuals of the same
species. When either case occurs, it would be easy fornatural selection to fit the animal, by some modificationof its structure, for its changed habits, or exclusively
for one of its several different habits. But it is difficult
to tell, and immaterial for us, whether habits generally
change first and structure afterwards ; or whether slight
modifications of structure lead to changed habits ; bothprobably often change almost simultaneously. Of casesof changed habits it will suffice merely to allude to thatof the many British insects which now feed on exotic
plants, or exclusively on artificial substances. Of diversified habits innumerable instances could be given: I
have often watched a tyrant flycatcher (Saurophagus
sulphuratus) in South America, hovering over one spot
and then proceeding to another, like a kestrel, and atother times standing stationary on the margin of water,
and then dashing like a kingfkher at a fish. I n ourown country the larger titmouse (Parus major) may be

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DIFFICULTIES ON THEORY.

CHAP. VI.

seen climbing branches, almost like a creeper ; it often,
like a shrike, kills small birds by blows on the head;
and I have many times seen and heard it hammering
the seeds of the yew on a branch, and thus breaking
them like a nuthatch. In North America the black bear
was seen by Hearne swimming for hours with widely
open mouth, thus catching, like a whale, insects in thewater. Even in so extreme tt case as this, if the supply
of insects were constant, and if better adapted competitors did not already exist in the country, I can see nodifficulty in it race of bears being rendered, by natural
selection, more and more aquatic in their structureand habits, with larger and larger mouths, till a creature
was produced as monstrous as a whale.
As we sometimes see individuals of a species following
habits widely different from those both of their own
species and of the other species of the same genus, wemight expect, on my theory, that such individuals
would occasionally have given rise to new species, having
anomalous habits, and with their structure either slightly
or considerably modified from that of their proper type.
And such instances do occur in nature. Can a morestriking instance of adaptation be given than that of awoodpecker for climbing trees and for seizing insects inthe chinks of the bark? Yet in North America there
are woodpeckers which feed largely on fruit, and otherswith elongated wings which chase insects on the wing ;and on the plains of La Plata, where not a tree grows,
there is a woodpecker, which in every essential part of its
organisation, even in its colouring, in the harsh tone ofits voice, and undulatory flight, told me plainly of its
close blood-relationship to our common species ; yet it
is a woodpecker which never climbs a tree!
Petrels are the most agrial and oceanic of birds, yetin the quiet Sounds of Tierra del Fuego, the Pufftinuria

CHAP. VI.

TRANSITIONAL HABITS.

185

berardi, in its general habits, in its astonishing power of
diving, its manner of swimming, and of flying when
unwillingly it takes flight, would be mistaken by any
one for an auk or grebe ; nevertheless, it is essentiallya petrel, but with many parts of its organisation profoundly modified. On tlie other hand, the acutest
observer by examining the dead body of the water-ouzel
mould never have suspected its sub-aquatic habits ; yetthis anomalous member of the strictly terrestrial thrushfamily wholly subsists by diving,-grasping the stones
with its feet and using its wings under water.
H e who believes that each being has been created as
we now see it, must occasionally have felt surprise when
he has met with an animal having habits and structure
not at all in agreement. What can be plainer thanthat the webbed feet of ducks and geese are formed
for swimming ? yet there are upland geese with webbed
feet which rarely or never go near the water ; and noone except Audubon has seen the frigate-bird, which
has all its four toes webbed, alight on the surface of thesea. On the other hand, grebes and coots are eminently aquatic, although their toes are only borderedby membrane. What seems plainer than that the long
toes of grallatores are formed for walking over swamps
and floating plants, yet tlie water-hen is nearly as aquaticas the coot ; and the landrail nearly as terrestrial as thequail or partridge. I n such cases, and many others
could be given, habits have changed without a corresponding change of structure. The webbed feet of theupland goose may be said to have become rudimentaryin function, though not in structure. I n the frigatebird, the deeply-scooped membrane between the toesshows that structure has begun to change.
H e wlio believes in separate and innumerable acts ofcreation will say, that in these cases it has pleased the

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DIFFICULTIES ON THEORY.

CHAP. VI.

Creator to cause a being of one type to take the placeof one of another type; but this seems to me only
restating the fact in dignified language. H e who believes in t,he struggle for existence and in the principleof natural selection, will acknowledge that every organic
being is constantly endeavouring to increase in numbers ;and that if any one being vary ever so little, either in
habits or structure, and thus gain an advantage oversome other inhabitant of the country, it will seize on
the place of that inhabitant, however different it may
be from its own place. Hence it will cause him nosurprise that there should be geese and frigate-birds
with webbed feet, either living on the dry land or mostrarely alighting on the water; that there should belong-toed corncrakes living in meadows instead of inswamps; that there should be woodpeckers where nota tree grows ; that there should be diving thrushes, andpetrels with the habits of auks.

Organs of extreme perfection and comp1a’cation.To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances,
for admitting different amounts of light, and for thecorrection of spherical and chromatic aberration, could
have been formed by natural selection, seems, I freely
confess, absurd in the highest possible degree. Yet
reason tells me, that if numerous gradations from aperfect and complex eye to one very imperfect andsimple, each grade being useful to its possessor, can beshown to exist; if further, the eye does vary ever soslightly, and the variations be inherited, which is certainly the case; and if any variation or modificationin the organ be ever useful to an animal under changing
conditions of life, then the difficulty of believing thata perfect and complex eye could be formed by natural

CHAP. VI.

OEGANS OF EXTREME PERFECTION.

187

selection, though insuperable by our imagination, canhardly be considered real. How a nerve comes to besensitive to light, hardly concerns us more than howlife itself first originated; but I may remark thatseveral facts make me suspect that any sensitive nerve
may be rendered sensitive to light, and likewise to thosecoarser vibrations of the air which produce sound.
In looking for the gradations by which an organ in
any species has been perfected, we ought to look exclusively to its lineal ancestors ; but this is scarcely everpossible, and we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to
see what gradations are possible, and for the chanceof some gradations having been transmitted from theearlier stages of descent, in an unaltered or little alteredcondition. Amongst existing Vertebrata, we find but asmall amount of gradation in the structure of the eye,and from fossil species we can learn nothing on thishead. I n this great class we should probably have todescend far beneath the lowest known fossiIiferous
stratum to discover the earlier stages, by which the eye
has been perfected.
I n the Articulata we can commence a series with an
optic nerve merely coated with pigment, and without
any other mechanism ; and from this low stage, numerous gradations of stmcture, branching off in two fundamentally different lines, can be shown to exist, until
we reach a moderately high stage of perfection. I n
certain crustaceans, for instance, there is a double
cornea, the inner one divided into facets, within eacho f which there is a lens shaped swelling. I n othercrustaceans the transparent cones which are coated by
pigment, and which properly act only by excluding
lateral pencils of light, are convex at their upper ends

185

DIFFICULTIES ON THEORY.

CHAP. VI.

and must act by convergence ; and at their lower ends
there s e e m to be an imperfect vitreous substance. With
these facts, here far too briefly and imperfectly given,
which show that there is much graduated diversity inthe eyes of living crustaceans, and bearing in mind how
small the number of living animals is in proportion to
those which have become extinct, I can see no very
great difficulty (not more than in the case of manyother structures) in believing that natural selection
has converted the simple apparatus of an optic nerve
merely coated with pigment and invested by transparent
membrane, into an optical instrument as perfect as ispossessed by any member of the great Articulate class.
He who will go thus far, if he find on finishing thistreatise that large bodies of facts, otherwise inexplicable, can be explained by the theory of descent, ought
not to hesitate to go further, and to admit that a structure even as perfect as the eye of an eagle might beformed by natural selection, although in this case hedoes not know any of the transitional grades. His
reason ought to conquer his imagination; though I
have felt the difficulty far too keenly to be surprisedat any degree of hesitation in extending the principleof natural selection to such startling lengths.
It is scarcely possible to avoid comparing the eye toa t.elescope. We know that this instrument has been
perfected by the long-continued efforts of the highest
human intellects; and we naturally infer that the eye
has been formed by a somewhat analogous process. But
may not this inference be presumptuous? Have weany right to assume that the Creator works by intellectual powers like those of man? If we must compare
the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with anerve sensitive to light beneath, and then suppose every

CHAP.VI.

ORGANS O F EXTREME PERFECTION.

189

part of this layer to be continually changing slowly indensity, so as to separate into layers of different densities and thicknesses, placed at different distances fromeach other, and with the surfaces of each layer slowly
changing in form. Further we must suppose that there
is a power always intently watching each slight accidental alteration in the transparent layers ; and carefully
selecting each a1teration which, under varied circumstances, may in any way, or in any degree, tend to produce a distincter image. We must suppose each newstate of the instrument to be multiplied by the million ;and each to be preserved till a better be produced, andthen the old ones to be destroyed. I n living bodies,variation will cause the slight alterations, generation
will multiply them almost idnitely, and natural selection will pick out with unerring skill each improvement.
Let this process go on for millions on millions of years ;and during each year on millions of individuals of manykinds; and may we not believe that a living optical
instrument might thus be formed as superior to one ofglass, as the works of the Creator are to those of man?
If it could be demonstrated that any complex organ
existed, which could not possibly have been formed bynumerous, successive, slight modikations, my theory
would absolutely break down. But I can find out nosuch case. No doubt many organs exist of which we
do not know the transitional grades, more especially if
we look to much-isolated species, round which, according to my theory, there has been much extinction. Or
again, if we look to an organ common to all the members of a large class, for in this latter case the organ
must have been first formed at an extremely remoteperiod, since which all the many members of the classhave been developed; and in order to discover the
early transitional grades t,hrough which the organ has

190

DIFFICULTIES ON THEORY.

CHAP. VI.

passed, we should have to look to very ancient ancestralforms, long since become extinct.
We should be extremely cautious in concluding thatan organ could not have been formed by transitional
gradations of some kind. Numerous cases could be given
amongst the lower animals of the same organ performingat the same time wholly distinct functions ; thus the alimentary canal respires, digests, and excretes in the larvaof the dragon-fly and in the fish Cobites. I n the Hydra,
the animal may be turned inside out, and the exterior
surface will then digest and the stomach respire. I nsuch cases natural selection might easily specialise, if
any advantage were thus gained, a part or organ, which
had performed two functions, for one function alone,
and thus wholly change its nat,ure by insensible steps.
Two distinct organs sometimes perform simultaneously
the same function in the same individual ; to give one
instance, there are fish with gills or branchiae that
breathe the air dissolved in the water, at the same timethat they breathe free air in their swimbladders, thislatter organ having a ductus pneumaticus for its supply,and being divided by highly vascular partitions. I nthese cases, one of the two organs might with ease bemodified and perfected so as to perform all the workby itself, being aided during the process of modificationby the other organ; and then this other organ might
be modified for some other and quite distinct purpose,or be quite obliterated.
The illustration of the swimbladder in fishes is a good
one, because it shows us clearly the highly important factthat an organ originally constructed for one purpose,
namely flotation, may be converted into one for a wholly
different purpose, namely respiration. The swimbladderhas, also, been worked in as an accessory to the auditoryorgans of certain fish, or, for I do not know which

CHAP. VI.

TRANSITIONS O F ORGIAh’S.

191

view is now generally held, a part of the auditory
apparatus has been worked in as a complement to theswimbladder. All physio1ogist.s admit that the swimbladder is homologous, or “ ideally similar,” in positionand structure with the lungs of the higher vertebrate
animals: hence there seems to me to be no great difficulty in believing that natural selection has actuallyconverted a swimbladder into a lung, or organ used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true lungs have descended by ordinary
generation from an ancient prototype, of which we know
nothing, furnished with a floating apparatus or swimbladder. We can thus, as I infer from Professor Owen’s
interesting description of these parts, understand thestrange fact that every particle of food and drink which
we swallow has to pass over the orifice of the trachea,with some risk of falling into the lungs, notwithstanding
the beautiful contrivance by which the glottis is closed.
I n the higher Vertebrata the branchi= have wholly disappeared-the slits on the sides of the neck and theloop-like course of the arteries still marking in the embryo their former position. But it is conceivable thatthe now utterly lost branchiae might have been gradually
worked in by natural selection for some quite distinct
purpose : in the same manner as, on the view entertainedby some naturalists that the branchiae and dorsal scalesof Annelids are homologous with the wings and wingcovers of insects, it is probable that organs which at avery ancient period served for respiration have been
actually converted into organs of flight.
In considering transitions of organs, it is so important
to bear in mind the probability of conversion from one
function to another, that I will give one more instance.
Pedunculated cirripedes have two minute folds of skin,

192

DIFFICULTIES ON THEORY.

CHAP. VI.

called by me the ovigerous frena, which serve, throughthe means of a sticky secretion, to retain the eggs until
they are hatched within the sack. These cirripedes
have no branchiae, the whole surface of the body andsack, including the small frena, serving for respiration.
The Balanidae or sessile cirripedes, on the other hand,
have no ovigerous frena, the eggs lying loose at thebottom of the sack, in the well-enclosed shell ; but they
have large folded branchiae. Now I think no one willdispute that the ovigerous frena in the one family arestrictly homologous with the branchiae of the otherfamily ; indeed, they graduate into each other. Therefore I do not doubt that little folds of skin, which originally served as ovigerous frena, but which, likewise,very slightly aided the act of respiration, have been
gradually converted by natural selection into branchiae,
simply through an increase in their size and the obliteration of their adhesive glands. If all pedunculated
cirripedes had become extinct, and they have aareadysuffered far more extinction than have sessile cirripedes,
who would ever have imagined that the branchiae in thislatter family had originally existed as organs for preventing the ova from being washed out of the sack?
Although we must be extremely cautious in concluding that any organ could not possibly have been
produced by successive transitional gradations, yet, undoubtedly, grave cases of difficulty occur, some of which
will be discussed in my future work.
One of the gravest is that of neuter insects, which
are often very differently constructed from either the
males or fertile females; but this case will be treatedof in the next chapter. The electric organs of fishes
offer another case of special difficulty ; it is impossible
to conceive by what steps these wondrous organs havebeen produced ; but, as Owen and others have remarked,

CHAP.VI.

TRANSITIONS OF ORGANS.

193

their intimate structure closely resembles that of common
muscle ; and as it has lately been shown that Rays havean organ closely analogous to the electric apparatus, andyet do not, as Matteuchi asserts, discharge any electricity, we must own that we are far too ignorant to argue
that no transition of any kind is possible.
The electric organs offer another and even more
serious difficulty ; for they occur in only about a dozen
fishes, of which several are widely remote in theiraffinities. Generally when the same organ appears inseveral members of the same class, especially if in
members having very different habits of life, we may
attribute its presence to inheritance from a common
ancestor ; and its absence in some of the meybers to
its loss through disuse or natural selection. But if theelectric organs had been inherited from one ancient
progenitor thus provided, we might have expebted that
all electric fishes would have been specially related toeach other. Nor does geology at all lead to the belief
that formerly most fishes had electric organs, whichmost of their modified descendants have lost. The
presence of luminous organs in a few insects, belonging to different families and orders, offers a parallelcase of difficulty. Other cases could be given ; for instance in plants, the very curious contrivance of a massof pollen-grains, borne on a foot-stalk with a sticky
gland at the end, is the same in Orchis and Asclepias,genera almost as remote as possible amongst flowering
plants. I n all these cases of two very distinct species furnished with apparently the same anomalousorgan, it should be observed that, although the generalappearance and function of the organ may be the same,p t some fundamental difference can generally be detected. I am inclined to believe that in nearly the same
way as two men have sometimes independently hit on

194

DIFFICULTIES ON THEORY.

CHAP.VI.

the very same invention, so natural selection, working
for the good of each being and taking advantage of
analogous variations, has sometimes modified in very
nearly the same manner two parts in two organic beings,
which owe but little of their structure in common toinheritance from the same ancestor.
Although in many cases it is most difficult to conjecture by what transitions an organ could have arrived
at its present state ; yet, considering that the proportionof living and known forms to the extinct and unknown
is very small, I have been astonished how rarely anorgan can be named, towards which no transitional
grade is known to lead. The truth of this remark is
indeed shown by that old canon in natural history of‘‘Natura non facit saltum.” We meet with this admission in the writings of almost every experienced naturalist; or, as Milne Edwards has well expressed it,
nature is prodigal in variety, but niggard in innovation.
Why, on the theory of Creation, should this be so?
Why should all the parts and organs of many independent beings, each supposed to have been separately
created for its proper place in nature, be 80 invariably
linked together by graduated steps? Why should not
Nature have taken a leap from structure to structure?
On the theory of natural selection, we can clearly
understand why she should not; for natural selection
can act only by taking advantage of slight, successive
variations; she can never take a leap, but must advance by the shortest and slowest steps.

Organs of little apparent importance.-As natural
selection acts by life and death,-by the preservation ofindividuals with any favourable variation, and by thedestruction of those with any unfavourable deviation of
structure,-1 have sometimes felt much diEculty inCHAP.VI.

ORGANS OF LITTLE IMPORTANCE.

195

understanding the origin of simple parts, of which theimportance does not seem sufficient to cause the preservation of successively varying individuals. I have sometimes felt as much difficulty, though of a very different
kind, on this head, as in the case of an organ as perfectand complex as the eye.
In the first place, we are much too ignorant in regard
to the whole economy of any one organic being, to say
what slight modifications would be of importance ornot. I n a former chapter I have given instances ofmost trifling characters, such as the down on fruit andthe colour of the flesh, which, from determining theattacks of insects or from being correlated with constitutional differences, might assuredly be acted on by
natural selection, The tail of the giraffe looks like anartificially constructed fly-flapper ; and it seems atdrst
incredible that this could have been adapted for its
present purpose by successive slight modifications, eachbetter and better, for so trifling an object as driving
away flies; yet we should pause before being too positive even in this case, for we know that the distribution
and existence of cattle and other animals in SouthAmerica absolutely depends on their power of resisting
the attacks of insects: so that individuals which couldby any means defend themselve8 from these small
enemies, would be able to range into new pastures andthus gain a great advantage. It is not that the larger
quadrupeds are actually destroyed (except in some rarecases) by the flies, but they are incessantly harassed andtheir strength reduced, so that they are more subject todisease, or not so well enabled in a coming dearth tosearch for food, or to escape from beasts of prey.
Organs now of trifling importance have probably insome cases been of high importance to an early progenitor, and, after having been slowly perfected at a

196

DIFFICULTIES ON THEORY.

CHAP.VI.

former period, have been transmitted in nearly the samestate, although now become of very slight use ; and any
actually injurious deviations in their structure will always
have been checked by natural selection. Seeing how
important an organ of locomotion the tail is in mostaquatic animals, its general presence and use for many
purposes in so many land animals, which in their lungs
or modified swimbladders betray their aquatic origin,may perhaps be thus accounted for. A well-developed
tail having been formed in an aquatic animal, it might
subsequently come to be worked in for all sorts of purposes, as a fly-flapper, an organ of prehension, or as an
aid in turning, as with the dog, though the aid must be
slight, for the hare, with hardly any tail, can double
quickly enough.
I n the second place, we may sometimes attribute
importance to characters which are really of very little
importance, and which have originated from quite
secondary causes, independently of natural selection.
We should remember that climate, food, &c., probably
have some little direct influence on the organisation;that characters reappear from the law of reversion ; that
correlation of growth will have had a most important
influence in modifying various structures ; and finally,that sexual selection will often have largely modified
the external characters of animals having a will, togive one male .an advantage in fighting with anotheror in charming the females. Moreover when a modification of structure has primarily arisen from the aboveor other unknown causes, it may at first have beenof no advantage to the species, but may subsequently
have been taken advantage of by the descendants ofthe species under new conditions of life and with newly
acquired habits.
To give a few instances to illustrate these latter

CHAP. VI.

ORGANS OF LITTLE IMPORTANCE.

197

remarks. If green woodpeckers alone had existed, andwe did not know that there were many black and piedkinds, I dare say that we should have thought that thegreen colour was a beautiful adaptation to hide thistree-frequenting bird from its enemies ; and consequently that it was a character of importance and might
have been acquired t,hrough natural selection ; as it is,I have no doubt that the colour is due to some quitedistinct cause, probably to sexual selection. h trailingbamboo in the Malay hrchipelego climbs the loftiest
trees by the aid of exquisitely constructed hooks clustered around the ends of the branches, and this contrivance, no doubt, is of the highest service to theplant; but as we see nearly similar hooks on many
trees which are not climbers, the hooks on the bamboo
may have arisen from unknown laws of growth, andhave been subsequently taken advantage of by theplant undergoing further modification and becoming aclimber. The naked skin on the head of a vulture is
generally looked at as a direct adaptation for wallowing
in putridity; and so it may be, or it may possibly be
due to the direct action of putrid matter; but weshould be very cautious in drawing any such inference,
when we see tha& the skin on the head of the cleanfeeding male turkey is likewise naked. The sutures inthe skulls of young mammals have been advanced as abeautiful adaptation for aiding parturition, and no doubt
they facilitate, or may be indispensable for this act;
but as sutures occur in the skulls of young birds andreptiles, which have only to escape from a broken egg,
we may infer that this structure has arisen from thelaws of growth, and has been taken advantage of in the
parturition of the higher animals.
We are profoundly ignorant of the causes producing
slight and unimportant variations ; and we are immedi-

198

DIFFICULTIES ON THEORY.

CUAP. VI.

ately made conscious of this by reflecting on the differences in the breeds of our domesticated animals in
different countries,-more especially in the less civilizedcountries where there has been but little artificial selection. Careful observers are convinced that a damp climate affects the growth of the hair, and that with thehair the horns are correlated. Mountain breeds always
differ from lowland breeds ; and a mountainous country
would probably affect the hind limbs from exercisingthem more, and possibly even the form of the pelvis;
and then by the law of homologous variation, the front
limbs and even the head would probably be affected.
The shape, also, 4f the pelvis might affect by pressure
the shape of the head of the young in the womb. The
laborious breathing necessary in high regions would, we
have some reason to believe, increase the size of thechest; and again correlation would come into play.
Animals kept by savages in different countries
often have to struggle for their own subsistence,
and would be exposed to a certain extent to natural
selection, and individuals with slightly different constitutions would succeed best under different climates ;and there is reason to believe that constitution andcolour are correlated. A good observer, also, states thatin cattle susceptibility to the attacks of flies is correlatedwith colour, as is the liability to be poisoned by certain
plants; so that colour would be thus subjected to theaction of natural selection. B u t we are far too
ignorant to speculate on the relative importance ofthe several known and unknown laws of variation;and I have here alluded to them only to show that,
if we are unable to account for the characteristic
differences of our domestic breeds, which neverthelesswe generally admit to have arisen through ordinary
generation, we ought not to lay too much stress on ourCHAP. VI.

ORGANS O F LITTLE IMPORTANCE.

199

ignorance of the precise cause of the slight analogous
differences between species. I might have adduced forthis same purpose the differences between the races ofman, which are so strongIy marked; I may add thatsome little light can apparently be thrown on the originof these differences, chiefly through sexual selection ofa particular kind, but without here entering on copious
details my reasoning would appear frivolous.
The foregoing remarks lead me to say a few words
on the protest lately made by some naturalists, against
the utilitarian doctrine that every detail of structure
has been produced for the good of its possessor. They
believe that very many structures have been created forbeauty in the eyes of man, or for mere variety. This
doctrine, if true, would be absolutely fatal to my theory.
Yet I fully admit that many structures are of no direct
use to their possessors. Physical conditions probably
have had some little effect on structure, quite independently of any good thus gained. Correlation of
growth has no doubt played a most important part, anda useful modification of one part will often have entailed on other parts diversified changes of no direct use.
So again characters which formerly were useful, or
which formerly had arisen from correlation of growth,or from other unknown cause, may reappear from thelaw of reversion, though now of no direct use. The
effects of sexual selection, when displayed in beauty tocharm the females, can be called useful only in rather
a forced sense. But by far the most important consideration is that the chief part of the organisation of
every being is simply due to inheritance; and consequently, though each being assuredly is well fitted forits place in nature, many structures now have no direct
relation to the habits of life of each species. Thus, we
can hardly believe that the webbed feet of the upland

200

DIFFICULTIES ON THEORY.

CHAP. VI.

goose or of the frigate-bird are of special use to thesebirds; we cannot believe that the same bones in thearm of the monkey, in the fore leg of the horse, in thewing of the bat, and in the flipper of the seal, are of
special use to these animals. We may safely attributethese sti-uctures to inheritance. But to the progenitor
of the upland goose and of the frigate-bird, webbed feetno doubt were as useful as they now are to the mostaquatic of existing birds. So we may believe that theprogenitor of the seal had not a flipper, but a foot withfive toes fitted for walking or grasping; and we may
further venture to believe that the several bones in thelimbs of the monkey, horse, and bat, which have been
inherited from a common progenitor, were formerly of
more special use to that progenitor, or its progenitors,
than they now are to these animals having such widely
diversified habits. Therefore we may infer that theseseveral bones might have been acquired throughnatural selection, subjected formerly, as now, to theseveral laws of inheritance, reversion, correlation of
growth, &c. Hence every detail of structure in every
living creature (making some little allowance for the
direct action of physical conditions) may be viewed,
either as having been of special use to some ancestralform, or as being now of special use to the descendantsof this form-either directly, or indirectly through thecomplex laws of growth.
Natural selection cannot possibly produce any modification in any one species exclusively for the good of
another species ; though throughout nature one species
incessantly takes advantage of, and profits by, the structure of another. But natural selection can and does
often produce structures for the direct injury of otherspecies, as we see in the fang of the adder, and in theovipositor of the ichneumon, by which its eggs are depo-

CHAP.VI.

WHAT NATURAL SELECTION CAN DO.

201

sited in the living bodies of other insects. If it could
be proved that any part of the structure of any one
species hrtd been formed for the exclusive good of
another species, it would annihilate my theory, for such
could not have been produced through natural selection. Although many statements may be found inworks on natural history to this effect, I cannot findeven one which seems to me of any weight. It is
admitted that the rattlesnake has a poison-fang for its
own defence and for the destruction of its prey; but
some authors suppose that at the same time this snake
is furnished with a rattle for its own injury, namely, towarn its prey to escape. I would almost as soon believethat the cat curls the end of its tail when preparing tospring, in order to warn the doomed mouse. But I havenot space here to enter on this and other such cases.
Natural selection will never produce in a being anything injurious to itself, for natural selection acts solely
by and for the good of each. No organ will be formed,
as Paley has remarked, for the purpose of causing painor for doing an injury to its possessor. If a fair balance
be struck between the good and evil caused by eachpart, each will be found on the whole advantageous.
After the lapse of time, under changing conditions oflife, if any part comes to be injurious, it will be modified ; or if it be not so, the being will become extinct,
as myriads have become extinct.
Natural selection tends only to make each organic
being a,s perfect as, or slightly more perfect than, theother inhabitants of the same country with which it has
to struggle for existence. And we see that this is thedegree of perfection attained under nature. The endemic productions of New Zealand, for instance, areperfect one compared with another; but they are now
rapidly yielding before the advancing legions of plants

202

DIFFICULTIES ON THEORY.

CHAP.VI.

and animals introduced from Europe. Natural selection
will not produce absolute perfection, nor do we always
meet, as far as we can judge, with this high standard
under nature. The correction for the aberration oflight is said, on high authority, not to be perfect evenin that most perfect organ, the eye. If our reason
leads us to admire with enthusiasm a multitude ofinimitable contrivances in nature, this same reason tells
us, though we may easily err on both sides, that someother contrivances are less perfect. Can we consider
the sting of the wasp or of the bee as perfect, which,when used against many attacking animals, cannot be
withdrawn, owing to the backward serratures, and so
inevitably causes the death of the insect by tearing out
its viscera?
If we look at the sting of the bee, as having originally existed in a remote progenitor as a boring andserrated instrument, like that in so many members of
the same great order, and which has been modified but
not perfected for its present purpose, with the poison
originally adapted to cause galls subsequently intensified, we can perhaps understand how it is that the useof the sting should so often cause the insect’s own
death: for if on the whole the power of stinging be
useful to the community, it will fulfil all the requirements of natural selection, though it may cause thedeath of some few members. If we admire the truly
wonderful power of scent by which the males of many
insects find their females, can we admire the production
for this single purpose of thousands of drones, which areutterly useless to the community for any other end,
and which are ultimately slaughtered by their industrious and sterile sisters? It may be difficult, but weought to admire the savage instinctive hatred of thequeen-bee, which urges her instantly to destroy theCHAP. VI.

SUMMARY.

203

young queens her daughters as soon as born, or toperish herself in the combat; for undoubtedly this is
for the good of the community; and maternal love or
maternal hatred, though the latter fortunately is mostrare, is all the same to the inexorable principle ofnatural selection. If we admire the several ingenious
contrivances, by which the flowers of the orchis and ofmany other plants are fertilised through insect agency,
can we consider as equally perfect the elaboration by
our fir-trees of dense clouds of pollen, in order that a
few granules may be wafted by a chance breeze on tothe ovules?
Summary of Chapter.-We have in this chapter discussed some of the difficulties and objections which may
be urged against my theory. Many of them are very
grave : but I think that in the discussion light has been
thrown on several facts, which on the theory of independent acts of creation are utterly obscure. We haveseen that species at any one period are not indefinitely
variable, and are not linked together by a multitudeof intermediate gradations, partly because the process ofnatural selection will always be very slow, and will act,
at any one time, only on a very few forms; and partly
because the very process of natural selection almost
implies the continual supplanting and extinction of preceding and intermediate gradations. Closely allied species, now living on a continuous area, must often havebeen formed when the area was not continuous, andwhen the conditions of life did not insensibly graduate
away from one part to another. When two varieties
are formed in two districts of a continuous area, an intermediate variety w i l l often be formed, fitted for anintermediate zone ; but from reasons assigned, the intermediate variety will usually exist in lesser numbers than

204

DIFFICULTIES ON THEORY.

CHAP. VI.

the two forms which it connects ; consequently the twolatter, during the course of further modification, fromexisting in greater numbers, will have a great advantage
over the less numerous intermediate variety, and willthus generally succeed in supplanting and exterminating it.
We have seen in this chapter how cautious we should
be in concluding that the most different habits of life
could not graduate into each other; that a bat, forinstance, could not have been formed by natural selection from an animal which at first could only glide
through the air.
We have seen that a species may under new conditions of life change its habits, or have diversified habits,with some habits very unlike those of its nearest congeners. Hence we can understand, bearing in mind
that each organic being is trying to live wherever it
can live, how it has arisen that there are upland geesewith webbed feet, ground woodpeckers, diving thrushes,and petrels with the habits of auks.
Although the belief that an organ so perfect as theeye could have been formed by natural selection, ismore than enough to stagger any one ; yet in the caseof any organ, if we know of a long series of gradations
in complexity, each good for its possessor, then, underchanging conditions of life, there is no logical impossibility in the acquirement of any conceivable degree of
perfection through natural selection. In the cases inwhich we know of no intermediate or transitional states,
we should be very cautious in concluding that none
could have existed, for the homologies of many organs
and their intermediate states show that wonderful metamorphoses in function are at least possible. For instance,a swim-bladder has apparently been converted into an
air-breathing lung. The =me organ having performed

CHAP. VI.

SUMMARY.

205

simultaneously very different functions, and then having
been specialised for one function ; and two very distinct
organs having performed at the same time the samefunction, the one having been perfected whilst aided
by the other, must often have largely facilitatedtransitions.
We are far too ignorant, in almost every case, to beenabled to assert that any part or organ is so unimportant for the welfare of a species, that modificationsin its structure could not have been slowly accumulated
by means of natural selection. But we may confidently
believe that many modifications, wholly due to the lawsof growth, and at first in no way advantageous to a species, have been subsequently taken advantage of by thestill further modified descendants of this species. We
may, also, believe that a part formerly of high importance has often been retained (as the tail of an aquatic
animal by its terrestrial descendants), though it has
become of such small importance that it could not, in
its present state, have been acquired by natural selection,-a power which acts solely by the preservation of
profitable variations in the struggle for life.
Natural selection will produce nothing in one species
for the exclusive good or injury of another ; though it
may well produce parts, organs, and excretions highly
useful or even indispensable, or highly injurious toanother species, but in all cases at the same time usefulto the owner. Natural selection in each well-stocked
country, must act chiefly through the competition of
the inhabitants one with another, and consequently willproduce perfection, or strength in the battle for life, only
according to the standard of that country. Hence theinhabitants of one country, generally the smaller one,will often yield, as we see they do yield, to the inhabitants of another and generally larger country. For in

206

DIFFICULTIES ON THEORY.

CHAP.VI.

the larger country there will have existed more individuals, and more diversified forms, and the competition
will have been severer, and thus the standard of pei-fection will have been rendered higher. Natural selection
w i l l not necessarily produce absolute perfection ; nor, as
far as we can judge by our limited faculties, can absolute
perfection be everywhere found.
On the theory of natural selection we can clearly
understand the full meaning of that old canon in natural
history, ‘‘ Natura non facit salturn.” This canon, if
we look only to the present inhabitants of the world, is
not strictly correct, but if we include all those of pasttimes, it must by my theory be strictly true.
It is generally acknowledged that all organic beings
have been formed on two great laws-Unity of Type,and the Conditions of Existence. By unity of type ismeant that fundamental agreement in structure, which
we see in organic beings of the same class, and which isquite independent of their habits of life. On my theory,
unity of type is explained by unity of descent. The
expression of conditions of existence, so often insisted onby the illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts byeither now adapting the varying parts of each being to
its organic and inorganic conditions of life ; or by having
adapted them during long-past periods of time: theadaptations being aided in some cases by use and disuse, being slightly affected by the direct action of theexternal conditions of life, and being in all cases subjected to the several laws of growth. Hence, in fact,
the law of the Conditions of Existence is the higher
law ; as it includes, through the inheritance of former
adaptations, that of Unity of Type.

CHAP. VII.

IKSTINCT.

207

C H A P T E R VII.
INSTINCT.
Instincts comparable with habits, but different in their originInstincts graduated - Aphides and ants -Instincts variableDomestic instincts, t.heir origin-Natural instincts of the cuckoo,
ostrich, and parasitic bees -Slave-making ants -Hive-bee, its
cell-making instinct-Difficulties on the theory of the Natural
Selection of instincts-Neuter or sterile insects-Summary.

THEsubject of instinct might have been worked into theprevious chapters ; but I have thought that it would be
more convenient to treat the subject separately, especially as so wonderful an instinct as that of the hivebee making its cells will probably have occurred tomany readers, as a difficulty sufficient to overthrow mywhole theory. I must premise, that I have nothing to
do with the origin of the primary mental powers, any
more than I have with that of life itself. We are concerned only with the diversities of instinct and of theother mental qualities of animals within the same class.
I will not attempt any definition of instinct. It would
be easy to show that several distinct mental actions arecommonly embraced by this term ; but every one understands what is meant, when it is said that instinct impels
the cuckoo to migrate and to lay her eggs in other birds’
nests. An action, which we ourselves should require
experience to enable us t.0 perform, when performed byan animal, more especially by a very young one, without
any experience, and when performed by many individuals in the same way, without their knowing for what
purpose it is performed, is usually said to be instinctive,

208

INSTINCT.

CHAP. VII.

But I could show that none of these characters of
instinct are universal. A little dose, as Pierre Huber
expresses it, of judgment or reason, often comes into
play, even in animals very low in the scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared instinct with habit. This comparison gives, I think, a remarkably accurate notion of
the frame of mind under which an instinctive action is
performed, but not of its origin. How unconsciously
many habitual actions are performed, indeed not rarely
in direct opposition to our conscious will! yet they may
be modified by the will or reason. Habits easily become
associated with other habits, and with certain periods oftime and states of the body. When once acquired, theyoften remain constant througliout life. Several otherpoints of resemblance between instincts and habits could
be pointed out. As in repeating a well-known song, so
in instincts, one action follows another by a sort ofrhythm; if a person be interrupted in a song, or in
repeating anything by rote, he is generally forced to go
back to recover the habitual train of thought: so P.
Huber found it was with a caterpillar, which makes avery complicated hammock ; for if he took a caterpillar
which had completed its hammock up to, say, the sixth
stage of construction, and put it into a hammock completed up only to the third stage, the caterpillar simply
re-performed the fourth, fifth, and sixth stages of construction. If, however, a caterpillar were taken out ofa hammock made up, for instance, to the third stage,
and were put into one finished up to the sixth stage, sothat much of its work was already done for it, far fromfeeling the benefit of this, it was much embarrassed, and,in order to complete its hammock, seemed forced tostart from the third stage, where it had left off, and thus
tried to complete the already fhished work.

CHAP. VII.

INSTINCT LIKE HABIT.

209

If we suppose any habitual action to become inherited-andI think it can be shown that this does
sometimes happen-then the resemblance between what
originally was a habit and an instinct becomes so close
as not to be distinguished. If Mozart, instead of playing
the pianoforte at three years old with wonderfully little
practice, had played a tune with no practice at all, hemight truly be said to have done so instinctively. But
it would be the most serious error to suppose that thegreater number of instincts have been acquired by habit
in one generation, and then transmitted by inheritanceto succeeding generations. It can be clearly shown thatthe most wonderful instincts with which we are acquainted, namely, those of the hive-bee and of many
ants, could not possibly have been thus acquired.
It will be universally admitted that instincts are
as important as corporeal structure for the welfareof each species, under its present conditions of life.
Under changed conditions of life, it is at least possiblethat slight modifications of instinct might be profitable
to a species; and if it can be shown that instincts dovary ever so little, then I can see no difficulty in natural
selection preserving and continually accumulating variations of instinct to any extent that may be profitable.
It is thus, as I believe, that all the most complex andwonderful instincts have originated. As modificationsof corporeal structure arise from, and are increased by,use or habit, and are diminished or lost by disuse, SO I do
not doubt it has been with instincts. But 1believe thatthe effects of habit are of quite subordinate importanceto the effects of the natural selection of what may becalled accidental variations of instincts ;-that is of variations produced by the same unknown causes which produce slight deviations of bodily structure.
No complex instinct can possibly be produced through

210

INSTINCT.

CHAP. VII.

natural selection, except by the slow and gradual accumulation of numerous, slight, yet profitable, variations.
Hence, as in the case of corporeal structures, we ought
to find in nature, not the actual transitional gradations
by which each complex instinct has been acquired-for
these could be found only in the lineal ancestors of eachspecies-but we ought to find in the collateral lines ofdescent some evidence of such gradations ; or we ought
at least to be able to show that gradations of some kindare possible; and this we certainly can do. I havebeen surprised to find, making allowance for the instinctsof animals having been but little observed except inEurope and North America, and for no instinct being
known amongst extinct species, how very generally gradations, leading to the most complex instincts, can be
discovered. The canon of “Natura non facit saltum”
applies with almost equal force to instincts as to bodily
organs. Changes of instinct may sometimes be facilitated by the same species having different instincts at
different periods of life, or at different seasons of theyear, or when placed under different circumstances, &c. ;in which case either one or the other instinct might
be preserved by natural selection. And such instancesof diversity of instinct in the same species can be shownto occur in nature.
Again as in the case of corporeal structure, and conformably with my theory, the instinct of each species isgood for itself, but has never, as far as we can judge,
been produced for the exclusive good of others. One of
the strongest instances of an animal apparently performing an action for the sole good of another, withwhich I am acquainted, is that of aphides voluntarily
yielding their sweet excretion to ants: that they do sovoluntarily, the following facts show. I removed all theant,s from a group of about a dozen aphides on a dock-CHAP. VII.

INSTINCT.

211

plant, and prevented their attendance during several
hours. After this interval, I felt sure that the aphides
would want to excrete. I watched them for some timethrough a lens, but not one excreted; I then tickled
and stroked them with a hair in the same manner, as
well as I could, as the ants do with their antennae ; but
not one excreted. Afterwards I allowed an ant to visit
them, and it immediately seemed, by its eager way ofrunning about, to be well aware what a rich flock it had
discovered; it then began to play with its antennae on
the abdomen first of one aphis and then of another;and each aphis, as soon as it felt the antennae, immediately lifted up its abdomen and excreted a limpid
drop of sweet juice, which was eagerly devoured by theant. Even the quite young aphides behaved in thismanner, showing that the action was instinctive, andnot the result of experience. But as the excretion isextremely viscid, it is probably a convenience to theaphides to have it removed ; and therefore probably theaphides do not instinctively excrete for the sole good of
the ants. Although I do not believe that any animal in
the world performs an action for the exclusive good of
another of a distinct species, yet each species tries totake advantage of the instincts of others, as each takes
advantage of the weaker bodily structure of others. So
again, in some few cases, certain instincts cannot be
considered as absolutely perfect ; but as details on thisand other such points are not indispensable, they may
be here passed over.
As some degree of variation in instincts under astate of nature, and the inheritance of such variations,
are indispensable for the action of natural selection, as
many instances as possible ought to have been here
given; but want of space prevents me. I can only
assert, that instincts certainly do vary-for instance,

212

INSTINCT.

CHAP. VII.

the migratory instinct, both in extent and direction,and in its total loss. So it is with the nests of birds,
which vary partly in dependence on the situations
chosen, and on the nature and temperature of thecountry inhabited, but often from causes wholly unknown to us: Audubon has given several remarkablecases of differences in nests of the same species in thenorthern and southern United States. Fear of any
particular enemy is certainly an instinctive quality, as
may be seen in nestling birds, though it is strengthenedby experience, and by the sight of fear of the same
enemy in other animals. But fear of man is slowly
acquired, as I have elsewhere shown, by various animals
inhabiting desert islands ; and we may see an instanceof this, even in England, in the greater wildness of
all our large birds than of our small birds; for thelarge birds have been most persecuted by man. We
may safely attribute the greater wildness of our large
birds to thi; cause ; for in uninhabited islands large
birds are not more fearful than small ; and the magpie,
so waiy in England, is tame in Norway, as is thehooded crow in Egypt.
That the general disposition of individuals of the same
species, born in a state of nature, is extremely diversified,can be shown by a multitude of facts. Several cases
also, could be given, of occasional and strange habits incertain species, which might, if advantageous to thespecies, give rise, through natural selection, t o quite new
instincts. But I am well aware that these general statements, without facts given in detail, can produce but afeeble effect on the reader’s mind. I can only repeat myassurance, that I do not speak without good evidence.
The possibility, or even probability, of inherited
variations of instinct in a state of nature will bestrengthened by briefly considering a few cases underCHAP. VII.

DOMESTIC INSTINCTS.

213

domestication. We shall thus also be enabled to see
the respective parts which habit and the selection of socalled accidental variations have played in modifying
the mental qualities of our domestic animals. A numberof curious and authentic instances could be given of
the inheritance of all shades of disposition and tastes,
and likewise of the oddest tricks, associated with certain
frames of mind or periods of time. But let us look to
the familiar case of the several breeds of dogs: itcannot be doubted that young pointers ( I have myself
seen a striking instance) will sometimes point and evenback other dogs the very first time that they are taken
out ; retrieving is certainly in some degree inherited byretrievers; and a tendency to run round, instead of at,
a flock of sheep, by shepherd-dogs. I cannot see thatthese actions, performed without experience by theyoung, and in nearly the same manner by each individual, performed with eager delight by each breed, andwithout the end being known,-for the young pointercan no more know that he points to aid his master,than the white butterfly knows why she lays her eggs
on the leaf of the cabbage,-I cannot see that theseactions differ essentially from true instincts. If wewere to see one kind of wolf, when young and without
any training, as soon as it scented its prey, stand motionless like a statue, and then slowly crawl forward with apeculiar gait ; and another kind of wolf rushing round,instead of at, a herd of deer, and driving them to a
distant point, we should assuredly call these actions instinctive. Domestic instincts, as they may be called,
are certainly far less fixed or invariable than natural
instincts ; but they have been acted on by far less rigorous selection, and have been transmitted for an incomparably shorter period, under less fixed conditions of life.
How strongly these domestic instincts, habits, and dis-214

INBTINCT.

CHAP.VII.

positions are inherited, and how curiously they become
mingled, is well shown when different breeds of dogs arecrossed. Thus it is known that a cross with a bull-dog
has affected for many generations the courage and obstinacy of greyhounds ; and a cross with a greyhound has
given to a whole family of shepherd-dogs a tendency tohunt hares. These domestic instincts, when thus tested
by crossing, resemble natural instincts, which in a likemanner become curiously blended together, and for along period exhibit traces of the instincts of either
parent: for example, Le Roy describes a dog, whose
great-grandfather was a wolf, and this dog showed a
trace of its wild parentage only in one way, by not
coming in a straight line to his master when called.
Domestic instincts are sometimes spoken of as actions
which have become inherited solely from long-continued
and compulsory habit, but this, I think, is not true.
No one would ever have thought of teaching, or probably could have taught, the tumbler-pigeon to tumble,an action which, as I have witnessed, is performed byyoung birds, that have never seen a pigeon tumble.
We may believe that some one pigeon showed a slight tendency to this strange habit, and that the long-continued
selection of the best individuals in successive generations
made tumblers what they now are; and near Glasgow
there are house-tumblers, as I hear from Mr. Brent,
which cannot fly eighteen inches high without goinghead over heels. It may be doubted whether any one
would have thought of training a dog to point, had notsome one dog naturally shown a tendency in this line ;and this is known occasionally to happen, as I once
saw in a pure terrier. When the first tendency wasonce displayed, methodical selection and the inheritedeffects of compulsory training in each successive generation would soon complete the work ; and unconscious

CHAP.VII.

DOMESTIC INSTINCTS.

215

selection is still at work, as each man tries to procure,without intending to improve the breed, dogs which w i l lstand and hunt best. On the other hand, habit alone
in some cases has sufficed; no animal is more diRicult
to tame than the young of the wild rabbit; scarcely
any animal is tamer than the young of the tame rabbit ;but I do not suppose that domestic rabbits have everbeen selected for tameness; and I presume that wemust attribute the whole of’ the inherited change fromextreme wildness to extreme tameness, simply to habit
and long-continued close confinement.
Natural instincts are lost under domestication : a remarkable instance of this is seen in those breeds offowls which very rarely or never become “ broody,”
that is, neTer wish to sit on their eggs. Familiarity
alone prevents our seeing how universally and largely
the minds of our domestic animals have been m o a e dby domestication. It is scarcely possible t o doubt thatthe love of man has become instinctive in the dog. Allwolves, foxes, jackals, and species of the cat genus, when
kept tame, are most eager to attack poultry, sheep, andpigs; and this tendency has been found incurable indogs which have been brought home as puppies fromcountries, such as Tierra del Fuego and Australia, where
the savages do not keep these domestic animals. How
rarely, on the other hand, do our civilised dogs, evenwhen quite young, require to be taught not to attack
poultry, sheep, and pigs! No doubt they occasionallydo make an attack, and are then beaten; and if not
cured, they are destroyed; so that habit, with somedegree of selection, has probably concurred in civilisingby inheritance our dogs. On the other hand, young
chickens have lost, wholly by habit, that fear of the dog
and cat which no doubt was originally instinctive inthem, in the same way as it is so plainly instinctive in

216

INSTINCT.

CHAP. VII.

young pheasants, though reared under a hen. It is notthat chickens have lost all fear, but fear only of dogs
and cats, for if the hen gives the danger-chuckle, they
will run (more especially young turkeys) from under
her, and conceal themselves in the surrounding grass or
thickets ; and this is evidently done for the instinctive
purpose of allowing, as we see in wild ground-birds, theirmother to fly away. But this instinct retained by ourchickens has become useless under domestication, forthe mother-hen has almost lost by disuse the power offlight.
Hence, we may conclude, that domestic instincts havebeen acquired and natural instincts have been lost partly
by habit, and partly by man selecting and accumulating
during successive ,generations, peculiar mental habits
and actions, which at first appeared from what we mustin our ignorance call an accident. In some cases compulsory habit alone has sufficed to produce such inherited mental changes ; in other cases compulsory habit
has done nothing, and all has been the result of selection, pursued both methodically and unconsciously ; but
in most cases, probably, habit and selection have acted
together.
We shall, perhaps, best understand how instincts in astate of nature have become modified by selection, byconsidering a few cases. I will select only three, out of
the several which I shall have to discuss in my future
work,-namely, the instinct which leads the cuckoo tolay her eggs in other birds’ nests; the slave-making
instinct of certain ants ; and the comb-making power ofthe hive-bee : these two latter instincts have generally,
and most justly, been ranked by naturalists as the mostwonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause of the cuckoo’s instinct is, that

CHAP. VII.

O F THE CUCKOO.

217

she lays her eggs, not daily, but at intervals of two or
three days ; so that, if she were to make her own nest
and sit on her own eggs, those first laid would haveto be left for some time unincubated, or there would be
eggs and young birds of different ages in the same nest.
If this were the case, the process of laying and hatchingmight be inconveniently long, more especially as she
has to migrate at a very early period; and the first
hatched young would probably have to be fed by themale alone. But the American cuckoo is in this predicament; for she makes her own nest and has eggs
and young successively hatched, all at the same time.
It has been asserted that the American cuckoo occasionally lays her eggs in other birds’ nests ; but I hearon the high authority of Dr. Brewer, that this is a mistake. Nevertheless, I could give several instances ofvarious birds which have been known occasionally to
lay their eggs in other birds’ nests. Now let us s u ppose that the ancient progenitor of our European
cuckoo had the habits of the American cuckoo; butthat occasionally she laid an egg in another bird’s
nest. If the old bird profited by this occasional habit,
or if the young were made more vigorous by advantage
having been taken of the mistaken maternal instinct ofanother bird, than by their own mother’s care, encumbered as she can hardly fail to be by having eggs andyoung of different ages at the same time ; then the oldbirds or the fostered young would gain an advantage.
And analogy would lead me to believe, that the young
thus reared would be apt to follow by iiiheritance theoccasional and aberrant habit of their mother, and intheir turn would be apt to lay their eggs in other birds‘
nests, and thus be successful in rearing their young.
By a continued process of this nature, I believe that thestrange instinct of our cuckoo could be, and has been,

228

INSTINCT.

CHAP. VII.

generated. I may add that, according to Dr. Grayand to some other observers, the European cuckoo has
not utterly lost all maternal love and care for her own
offspring.
The occasional habit of birds laying their eggs inother birds’ nests, either of the same or of a distinct
species, is not very uncommon with the Gallinaceae;
and this perhaps explains the origin of a singularinstinct in the allied group of ostriches. For several
hen ostriches, at least in the case of the American
species, unite and lay first a few eggs in one nest andthen in another; and these are hatched by the males.
This instinct may probably be accounted for by the factof the hens laying a large number of eggs ; but, as in
the case of the cuckoo, at intervals of two or three days.
This instinct, however, of the American ostrich has notas yet been perfected ; for a surprising number of eggs
lie strewed over the plains, so that in one day’s hunting
I picked up no less than twenty lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in
the nests of bees of other kinds. This case is more remarkable than that of the cuckoo ; for these bees have
not only their instincts but their structure modified inaccordance with their parasitic habits ; for they do notpossess the pollen-collecting apparatus which would benecessary if they had to store food for their own young.
Some species, likewise, of Sphegidae (wasp-like insects)
are parasitic on other species ; and M. Fabre has lately
shown good reason for believing that although the
Tachytes nigra generally makes its own burrow andstores it with paralysed prey for its own larvae to feedon, yet that when this insect finds a burrow alreadymade and stored by another sphex, it takes advantageof the prize, and becomes for the occasion parasitic. In
this case, as with the supposed case of the cuckoo, I canCHAP. VII.

SLAVE-MAKING INSTINCT.

219

see no difficulty in natural selection making an occasional habit permanent, if of advantage to the species,
and if the insect whose nest and stored food are thus
feloniously appropriated, be not thus extermbated.

Slave-making instinct.-This remarkable instinct was
first discovered in the Formica (Polyerges) rufescens
by Pierre Huber, a better observer even than his celebrated father. This ant is absolutely dependent on its
slaves ; without their aid, the species would certainly
become extinct in a single year. The males and fertile
females do no work. The workers or sterile females,
though most energetic and courageous in capturing
slaves, do no other work. They are incapable ofmaking their own nests, or of feeding their own
larvae. When the old nest is found inconvenient, andthey have to migrate, it is the slaves which determine
the migration, and actually carry their masters in theirjaws. So utterly helpless are the masters, that when
Huber shut up thirty of them without a slave, but withplenty of the food which they like best, and with theirlarvae and pupae to stimulate them to work, they did
nothing; they could not even feed themselves, andmany perished of hunger. Huber then introduced asingle slave (F. fusca), and she instantly set to work,
fed and saved the survivors; made some cells andtended the larvae, and put all to rights. What can be
more extraordinary than these well-ascertained facts ?
I f we had not known of any other slave-making ant, it
would have been hopeless to have speculated how sowonderful an instinct could have been perfected.
Formica sanguinea was likewise first discovered by
P. Huber to be a slave-making ant. This species isfound in the southern parts of England, and its habits
have been attended to by Mr. F. Smith, of the British

220

INSTINCT.

CHAP.VII.

Museum, to whom I am much indebted for informationon this and other subjects. Although fully trusting to
the statements of Huber and Mr. Smith, I tried toapproach the subject in a sceptical frame of mind, as
any one may well be excused for doubting the truth of
so extraordinary and odious an instinct as that ofmaking slaves. Hence I will give the observations which
I have myself made, in some little detail. I opened
fourteen nests of F. sanguinea, and found a few slaves inall. Males and fertile females of the slave-species arefound only in their own proper communities, and havenever been observed in the nests of F. sanguinea. The
slaves are black and not above half the size of their red
masters, so that the contrast in their appearance is very
great. When the nest is slightly disturbed, the slavesoccasionally come out, and like their masters are muchagitated and defend the nest : when the nest is much
disturbed and the larvae and pupae are exposed, theslaves work energetically with their masters in carrying
them away to a place of safety. Hence, it is clear,that the slaves feel quite at home. During the monthsof June and July, on three successive years, I havewatched for many hours several nests in Surrey and
Sussex, and never saw a slave either leave or entera nest. As, during these months, the slaves are very
few in number, I thought that they might behave differently when more numerous ; but Mr. Smith informs methat he has watched the nests at various h o y s during
May, June and August, both in Surrey and Hampshire,and has never seen the slaves, though present in large
numbers in August, either leave or enter the nest.
Hence he considers them as strictly household slaves.
The masters, on the other hand, may be constantly
seen bringing in materials for the nest, and food of all
kinds. During the present year, however, in the monthCHAP. VII.

SLAVE-MAKING INSTINCT.

221

of July, I came across a community with an unusuallylarge stock of slaves, and I observed a few slaves mingledwith their masters leaving the nest, and marching along
the same road to a tall Scotch-fir-tree, twenty-five yards
distant, which they ascended together, probably in searchof aphides or cocci. According to Hubcr, who had ample
opportunities for observation, in Switzerland the slaves
habitually work with their masters in making the nest,
and they alone open and close the doors in the morningand evening; and, as Huber expressly states, theirprincipal office is to search for aphides. This difference in the usual habits of the masters and slavesin the two countries, probably depends merely on theslaves being captured in greater numbers in Switzerland
than in England.
One day I fortunately chanced to witness a migrationfrom one nest to another, and it was a most interesting
spectacle to behold the masters carefully carrying, as
Huber has described, their slaves in their jaws. Anotherday my attention was struck by about a score of theslave-makers haunting the same spot, and evidently not
in search of food ; they approached and were vigorously
repulsed by an independent community of the slave
species (F. fusca) ; sometimes as many as three of theseants clinging to the legs of the slave-making F. sanguinea. The latter ruthlessly killed their small opponents, and carried their dead bodies as food to theirnest, twenty-nine yards distant ; but they were prevented from getting any pup% to rear as slaves. I
then dug up a small parcel of the pupae of F. fusca
from another nest, and put them down on a bare spotnear the place of combat; they were eagerly seized,
and carried off by the tyrants, who perhaps fancied
that, after all, they had been victorious in their latecombat.

222

INSTINCT.

CHAP. VII.

At the =me time I laid on the same place a small
parcel of the pupae of another species, F. flava, with afew of these little yellow ants still clinging to the fragments of the nest. This species is sometimes, thoughrarely, made into slaves, as has been described by Mr.
Smith. Although so small a species, it is very courageous, and I have seen it ferociously attack other ants.
In one instance I found to my surprise an independentcommunity of F. flava under a stone beneath anest of the slave-making F. sanguinea; and when I
had accidentally disturbed both nests, the little ants
attacked their big neighbours with surprising courage.
Now I was curious to ascertain whether F. sanguinea
could distinguish the pupae of F. fusca, which they
habitually make into slaves, from those of the little andfurious F. flava, which they rarely capture, and it wasevident that they did at once distinguish them : for wehave seen that they eagerly and instantly seized thepupae of F. fusca, whereas they were much terrified
when they came across the pupae, or even the earth
from the nest of F. flava, and quickly ran away ; but inabout a quarter of an hour, shortly after all the little
yellow ants had crawled away, they took heart and carried off the pupae.
One evening I visited another community of F. sanguinea, and found a number of these ants entering theirnest, carrying the dead bodies of F. fusca (showing that
it was not a migration) and numerous pupae. I traced
the returning file burthened with booty, for about forty
yards, to a very thick clump of heath, whence I saw thelast individual of F. sanguinea emerge, carrying a pupa ;but I was not able to find the desolated nest in the
thick heath. The nest, however, must have been closeat hand, for two or three individuals of F. fusca wererushing about in the greatest agitation, and one was

CHAP. VII.

SLAVE-MAKING INSTINCT.

223

perched motionless with its own pupa in its mouth on
the top of a spray of heath over its ravaged home.
Such are the facts, though they did not need co&rmation by me, in regard to the wonderful instinct ofmaking slaves. Let it be observed what a contrast theinstinctive habits of F. sanguinea present with those of
the F. rufescens. The latter does not build its own nest,
does not determine its own migrations, does not collect
food for itself or its young, and cannot even feeditself: it is absolutely dependent on its numerous sIaves
Formica sanguinea, on the other hand, possesses much
fewer slaves, and in the early part of the summer extremely few. The masters determine when and wherea new nest shall be formed, and when they migrate, themasters carry the slaves. Both in Switzerland andEngland the slaves seem to have the exclusive care of
the larvae, and the masters alone go on slave-making
expeditions. I n Switzerland the slaves and masters
work together, making and bringing materials for thenest : both, but chiefly the slaves, tend, and milk as it
may be called, their aphides ; and thus both collect foodfor the community. I n England the masters alone
usually leave the nest to collect building materials andfood for themselves, their slaves and larvae. So that themasters in this country receive much less service fromtheir slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated
I will not pretend to conjecture. But as ants, which are
not slave-makers, will, as I have seen, carry off pupae ofother species, if scattered near their nests, it is possiblethat pupa originally stored as food might become developed ; and the ants thus unintentionally reared would
then follow their proper instincts, and do what work
they could. If their presence proved useful to thespecies which had seized them-if it were more advan-

224

INSTINCT.

CHAP.1'11,

tageous to this species to capture workers than to procreate them-the habit of collecting pupae originally forfood might by natural selection be strengthened andrendered permanent for the very different purpose ofraising slaves. When the instinct was once acquired,
if carried out to a much less extent even than in our
British F. sanguinea, which, as we have seen, is lessaided by its slaves than the same species in Switzerland,
I can see no difficulty in natural selection increasing andmodifying the instinct-always supposing each modification to be of use to the species-until an ant mias
formed as abjectly dependent on its slaves as is theFormica rufescens.
Cell-making instinct of the Hive-Bee.--I will not here
enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived.
He must be a dull man who can examine the exquisite
structure of a comb, so beautifully adapted to its end,
without enthusiastic admiration. We hear from mathematicians that bees have practically solved a recondite
problem, and have made their cells of the proper shape
to hold the greatest possible amount of honey, with theleast possible consumption of precious wax in their construction. It has been remarked that a skilful workman, with fitting tools and measures, would find it very
difficult to make cells of wax of the true form, though
this is perfectly effected by a crowd of bees working ina dark hive. Grant whatever instincts you please, and it
seems at first quite inconceivable how they can make all
the necessary angles and planes, or even perceive when
they are correctly made. But the difficulty is notnearly so great as it at first appears: all this beautiful
work can be shown, I think, to follow from a few very
simple instincts.

CHAP.VII.

CELLS OF THE HIVE-BEE.

225

I was led to investigate this subject by Mr. Waterhouse, who has shown that the form of the cell stands
in close relation to the presence of adjoining cells ; andthe following view may, perhaps, be considered only asa modification of his theory. Let us look to the great
principle of gradation, and see whether Nature does notreveal to us her method of work. At one end of a shortseries we have humble-bees, which use their old cocoons
to hold honey, sometimes adding to them short tubes ofwax, and likewise making separate and very irregularroundedcells of wax. At the other end of the series wehave the cells of the hive-bee, placed in a double layer :each cell, as is well known, is an hexagonal prism, withthe basal edges of its six sides bevelled so as to join on
to a pyramid, formed of three rhombs. These rhombs
have certain angles, and the three which form the pyramidal base of a single cell on one side of the comb, enter
into the composition of the bases of three adjoining cellson the opposite side. I n the series between the extreme
perfection of the cells of the hive-bee and the simplicityof those of the humble-bee, we have the cells of the
Mexican Melipona dornestica, carefully described andfigured by Pierre Huber. The Melipona itself is intermediate in structure between the hive and humble bee,but more nearly related to the latter: it forms a nearly
regular waxen comb of cylindrical cells, in which theyoung are hatched, and, in addition, some large cells of
wax for holding honey. These latter cells are nearly
spherical and of nearly equal sizes, and are aggregatedinto an irregular mass. But the important point to
notice, is that these cells are always made at that
degree of nearness to each other, that they would haveintersected or broken into each other, if the spheres had
been completed ; but this is never permitted, the bees
building perfectly flat walls of wax between the spheres

226

INSTINCT.

CHAP. VII.

which thus tend to intersect. Hence each cell consistsof an outer spherical portion and of two, three, ormore perfectly flat surfaces, according as the cell adjoins two, three, or more other cells. When one cellcomes into contact with three other cells, which, fromthe spheres being nearly of the same size, is veryfrequently and necessarily the case, the three flat surfaces are united into a pyramid; and this pyramid, as
Huber has remarked, is manifestly a gross imitation of
the three-sided pyramidal basis of the cell of the hivebee. As in the cells of the hive-bee, so here, the threeplane surfaces in any one cell necessarily enter into theconstruction of three adjoining cells. It is obvious that
the Melipona saves wax by this manner of building ; forthe flat walls between the adjoining cells are not double,
but are of the same thickness as the outer sphericalportions, and yet each flat portion forms a part of two
cells.
Reflecting on this case, it occurred to me that if theMelipona had made its spheres at some given distancefrom each other, and had made them of equal sizes andhad arranged them symmetrically in a double layer, theresulting structure would probably have been as perfectas the comb of the hive-bee. Accordingly I wrote toProfessor Miller, of Cambridge, and this geometer has
kindly read over the following statement, drawn up fromhis information, and tells me that it is strictly correct :If a number of equal spheres be described with theircentres placed in two parallel layers ; with the centreof each sphere at the distance of radius x J 2, or
radius x 1.41421 (or at some lesser distance), from thecentres of the six surrounding spheres in the samelayer ; and at the same distance from the centres of theadjoining spheres in the other and parallel layer ; then,if planes of intersection between the several spheres inCHAP.VII.

CELL8 OF THE HIVE-BEE.

227

both layers be formed, there will result a double layer ofhexagonal prisms united together by pyramidal bases
formed of three rhombs ; and the rhombs and the sidesof the hexagonal prisms will have every angle identically the same with the best measuremenk which havebeen made of the cells of the hive-bee.
Hence we may safely conclude that if we could
slightly modify the instincts already possessed by the
Melipona, and in themselves not very wonderful, thisbee would make a structure as wonderfully perfect asthat of the hive-bee. We must suppose the Melipom
to make her cells truly spherical, and of equal sizes;
and this would not be very surprising, seeing that she
already does so to a certain extent, and seeing whatperfectly cylindrical burrows in wood many insects canmake, apparently by turning round on a iixed point.
We must suppose the Melipona to arrange. her cells inlevel layers, as she already does her cylindrical cells;
and we must further suppose, and this is the greatestdifficulty, that she can somehow judge accurately atwhat distance to stand from her fellow-labourers whenseveral are making their spheres ;-but she is alreadyso far enabled to judge of distance, that she always
describes her spheres so as to intersect largely; andthen she unites the points of intersection by perfectly
flat surfaces. We have further to suppose, but this is
no difficulty, that after hexagonal prisms have been
formed by the intersection of adjoining spheres in thesame layer, she can prolong the hexagon to any length
requisite to hold the stock of honey; in the same
way as the rude humble-bee adds cylinders of wax
to the circular mouths of her old cocoons. By suchmodifications of instincts in themselves not very wonderful,-hardly more wonderful than those which guidea bird to make its nest,-I believe that the hive-bee

228

INSTINCT.

CHAP. VII.

has acquired, through natural selection, her inimitablearchit,ectural powers.
But this theory can be tested by experiment. Following the example of Mr. Tegetmeier, I separated two
combs, and put between them a long, thick, square stripof wax: the bees instantly began to excavate minutecircular pits in i t ; and as they deepened these little
pits, they made them wider and wider until they wereconverted into shallow basins, appearing to the eye perfectly true or parts of a sphere, and of about the diameter of a cell. It was most interesting to me to observe that wherever several bees had begun to excavatethese basins near together, they had begun their workat such a distance from each other, that by the time thebasins had acquired the above stated width (i. e. about
the width of an ordinary cell), and were in depth about
one sixth of the diameter of the sphere of which they
formed a part, the rims of the basins intersected or
broke into each other. As soon as this occurred, thebees ceased to excavate, and began to build up flat wallsof wax on the lines of intersection between the basins,
so that each hexagonal prism was built upon the festooned edge of a smooth basin, instead of on the straightedges of a three-sided pyramid as in the case of ordinary
cells.
I then put into the hive, instead of a thick, squarepiece of wax, a thin and narrow, knife-edged ricge,
coloured with vermilion. The bees instantly began on
both sides to excavate little basins near to each other, inthe same way as before; but the ridge of wax was so
thin, that the bot,toms of the basins, if they had been
excavated to the same depth as in the former experiment, would have broken into each other from the
opposite sides. The bees, however, did not suffer thisto happen, andl they stopped their excavations in due

CHAP. 1'11.

CELLS O F THE HIVE-BEE.

229

t,ime; so that the basins, as soon as they had been alittle deepened, came to have flat bottoms ; and these flatbottoms, formed by thin little plates of the vermilion
wax having been left ungnawed, were situated, as far
as the eye could judge, exactly along the planes ofimaginary intersection between the basins on the opposite sides of the ridge of wax. I n parts, only little
bits, in other parts, large portions of a rhombic plate
had been left between the opposed basins, but the vork,from the unnahral state of things, had not been neatly
performed. The bees must have worked at very nearly
the same rate on the opposite sides of the ridge of vermilion wax, as they circularly gnawed away and deepened the basins on both sides, in order to have succeededin thus leaving flat plates between the basins, bystopping work along the intermediate planes or planesof intersection.
Considering how flexible thin wax is, I do not seethat there is any difficulty in the bees, whilst at work on
the two sides of a strip of wax, perceiving when they
have gnawed the wax away to the proper thinness,and then stopping their work. I n ordinary combs it
has appeared to me that the bees do not always succeed
in working at exactly the same rate from the opposite
sides ; for I have noticed half-completed rhombs at thebase of a just-commenced cell, which were slightly concave on one side, where I suppose that the bees had excavated too quickly, and convex on the opposed side,
where the bees had worked less quickly. I n one wellmarked instance, I put the comb back into the hive, andallowed the bees to go on working for a short time, andagain examined the cell, and I found that the rhombic
plate had been completed, and had becomeperfectlyflat :it was absolutely impossible, from the extreme thinnessof the little rhombic plate, that they could have effected

230

INSTINCT.

CHAP. VII.

this by gnawing away the convex side ; and I suspectthat the bees in such cases stand in the opposed cells
and push and bend the ductile and warm wax (whichas I have tried is easily done) into its proper intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax,
we can clearly see that if the bees were to build forthemselves a thin wall of wax, they could make theircells of the proper shape, by standing at the proper distance from each other, by excavating at the same rate,
and by endeavouring to make equal spherical hollows,
but never allowing the spheres to break into eachother. Now bees, as may be clearly seen by examining
the edge of a growing comb, do make a rough, circumferential wall or rim all round the comb; and they
gnaw into this from the opposite sides, always working
circularly as they deepen each cell. They do not makethe whole three-sided pyramidal base of any one cell at
the same time, but only the one rhombic plate which
stands on the extreme growing margin, or the two plates,as the case may be ; and they never complete the upperedges of the rhombic plates, until the hexagonal walls
are commenced. Some of these statements differ fromthose made by the justly celebrated elder Huber,
but I am convinced of their accuracy; and if I had
space, I could show that they are conformable with mytheory.
Huber’s statement tha.t the very first cell is excavatedout of a little parallel-sided wall of wax, is not, as far as
I have seen, strictly correct ; the first commencement
having always been a little hood of wax; but I will
not here enter on these details. We see how importanta part excavation plays in the construction of the cells ;but it would be a great error to suppose that the bees
cannot build up a rough wall of wax in the proper

CHAP. vIr.

CELLS OF THE HIVE-BEE.

231

position-that is, along the plane of intersection between
two adjoining spheres. I have several specimens showing clearly that they can do this. Even in the rude
circumferential rim or wall of wax round a growing
comb, flexures may sometimes be observed, corresponding in position to the planes of the rhombic basal platesof future cells. But the rough wall ofwax has in every
case to be finished off, by being largely gnawed away
on both sides. The manner in which the bees build ie
curious; they alwaysmake the first rough wall from ten
to twenty times thicker than the excessively thin finishedwall of the cell, which will ultimately be left. We shallunderstand how they work, by supposing masons first to
pile up a broad ridge of cement, and then to begin cutting
it away equally on both sides near the ground, till asmooth, very thin wall is left in the middle ; the masons
always piling up the cut-away cement, and adding fresh
cement, on the summit of the ridge. We shall thus
have a thin wall steadily growing upward ; but always
crowned by a gigantic coping. From all the cells,
both those just commenced and those completed, being
thus crowned by a strong coping of wax, the bees cancluster and crawl over the comb without injuring thedelicate hexagonal walls, which are only about one fourhundredth of an inch in thickness; the plates of the
pyramidal Oasis being about twice as thick. By this singular manner of building, strength is continually given
to the comb, with the utmost ultimate economy of wax.
It seems at first to add to the difficulty of understanding how the cells are made, that a multitude of bees all
work together ; one bee after working a short time at
one cell going to another, so that, as Huber has stated,a score of individuals work even at the commencementof the first cell. I was able practically to show thisfact, by covering the edges of the hexagonal walls

232

INSTINCT.

CHAP. VII.

of a single cell, or the extreme margin of the circumferential rim of a growing comb, with an extremely thinlayer of melted vermilion wax ; and I invariably foundthat the colour was most delicately diffused by the bees
-as delicately as a painter could have done with hisbrush-by atoms of the coloured wax having been takenfrom the spot on which it had been placed, and worked
into the growing edges of the cells all round. The workof construction seems to be a sort of balance struck
between many bees, all instinctively standing at thesame relative distance from each other, all trying to
sweep equal spheres, and then building up, or leaving
ungnawed, the planes of intersection between these
spheres. It was really curious to note in cases of difficulty, as when two pieces of comb met at an angle, howoften the bees would entirely pull down and rebuild in
different ways the same cell, sometimes recurring to ashape which they had at fist rejected.
When bees have a place on which they can stand intheir proper positions for working,-for instance, on aslip of wood, placed directly under the middle of a comb
growing downwards so that the comb has to be built over
one face of the slip-in this case the bees can lay thefoundations of one wall of a new hexagon, in its strictly
proper place, projecting beyond the other completed
cells. I t suffices that the bees should be enabled tostand at their proper relative distances from each otherand from the walls of the last completed cells, and then,by striking imaginary spheres, they can build up a wallintermediate between two adjoining spheres ; but, as far
as I have seen, they never gnaw away and finish off theangles of a cell till a large part both of that cell and of
the adjoining cells has been built. This capacity inbees of laying down under certain circumstances arough wall in its proper place between two just-com-

CHAP.VII.

CELLS OF THE HIVE-BEE.

233

menced cells, is important, as it bears on a fact,which seems at first quite subversive of the foregoingtheory; namely, that the cells on the extreme margin of
wasp-combs are sometimes strictly hexagonal ; but I
have not space here to enter on this subject. Nor does
there seem to me any great difficulty in a single insect(as in the case of a queen-wasp) making hexagonal cells,
if she work alternately on the inside and outside of twoor three cells commenced at the same time, always
standing at the proper relative distance from the partsof the cells just begun, sweeping spheres or cylinders,and building up intermediate planes. It is even conceivable that an insect might, by fixing on a point at whichto commence a cell, and then moving outside, first to
one point, and then to five other points, at the proper
relative distances from the central point and from eachother, strike the planes of intersection, and so make anisolated hexagon: but I am not aware that any suchcase has been observed ; nor would any good be derived
from a single hexagon being built, as in its construction
more materials would be required than for a cylinder.
As natural selection acts only by the accumulation ofslight modifications of structure or instinct, each profitable to the individual under its conditions of life, it may
reasonably be asked, how a long and graduated successionof modified architectural instincts, all tending towards thepresent perfect plan of construction, could have profited
the progenitors of the hive-bee? I think the answer is
not difficult : it is known that bees are often hard pressedto get sufficient nectar; and I am informed by Mr.
Tegetmeier that it has been experimentally found thatno less than from twelve to fifteen pounds of dry sugar
are consumed by a hive of bees for the secretion of eachpound of wax; so that a prodigious quantity of fluid nectar
must be collected and consumed by the bees in a hive for

234

INSTINCT.

CHAP. VII.

the secretion of the wax necessary for the construction of
their combs. Moreover, many bees have to remain idle
for many days during the process of secretion. A large
store of honey is indispensable to support a large stockof bees during the winter ; and the security of the hive
is known mainly to depend on a large number of bees
being supported. Hence the saving of wax by largely
saving honey must be a most important element of success in any family of bees. Of course the success of any
species of bee may be dependent on the number of its
parasites or other enemies, or on quite distinct causes,
and so be altogether independent of the quantity ofhoney which the bees could collect. But let us supposethat this latter circumstance determined, as it probably
often does determine, the numbers of a humble-bee which
could exist in a country; and let us further supposethat the community lived throughout the winter, and consequently required a store of honey: there can in thiscase be no doubt that it would be an advantage to ourhumble-bee, if a slight modification of her instinct led
her to make her waxen cells near together, so as tointersect a little; for a wall in common even to two
adjoining cells, would save some little wax. Hence itwould continually be more and more advantageous toour humble-bee, if she were to make her cells more andmore regular, nearer together, and aggregated into a
mass, like the cells of the Melipona; for in this casea large part of the bounding surface of each cell would
serve to bound other cells, and much wax would besaved. Again, from the same cause, it would be
advantageous to the Melipona, if she were to make hercells closer together, and more regular in every way
than at present; for then, as we have seen, thespherical surfaces would wholly disappear, and would
all be replaced by plane surfaces; and the Melipona

CRAF'. VII.

NEUTER INSECTS.

235

would make a comb as perfect as that of the hive-bee.
Beyond this stage of perfection in architecture, natural
selection could not lead; for the comb of the hivebee, as far as we can see, is absolutely perfect in economising wax.
Thus, as I believe, the most wonderful of all known
instincts, that of the hive-bee, can be explained bynatural selection having taken advantage of numerous,
successive, slight modi3cations of simpler instincts ;natural selection having by slow degrees, more andmore perfectly, led the bees to sweep equal spheresat a given distance from each other in a double
layer, and to build up and excavate the wax along
the planes of intersection. The bees, of course, nomore knowing that they swept their spheres at one
particular distance -from each other, than they knowwhat are the several angles of the hexagonal prisms
and of the basal rhombic plates. The motive powerof the process of natural selection having been economy
of wax; that individual swarm which wasted least
honey in the secretion of wax, having succeeded best,and having transmitted by inheritance its newly acquired
economical instinct to new swarms, which in their turnwill have had the best chance of succeeding in the
struggle for existence.

No doubt many instincts of very difficult explanation
could be opposed to the theory of natural selection,-cases, in which we cannot see how an instinct could
possibly have originated; cases, in which no intemediate gradations are known to exist; cases of instinct
of apparently such trifling importance, that they could
hardly have been acted on by natural selection ; cases of
instincts almost identically the same in animals so remote in the scale of nature, that we cannot account

236

INSTINCT.

CHAP. VII.

for their similarity by inheritance from a common
parent, and must therefore believe that they havebeen acquired by independent acts of natural selection.
I will not here enter on these several cases, but will
confine myself to one special difficulty, which at fistappeared to me insuperable, and actually fatal to mywhole theory. I allude to the neuters or sterile females
in insect-communities : for these neuters often differ
widely in instinct and in structure from both the males
and fertile females, and yet, from being sterile, theycannot propagate their kind.
The subject well deserves to be discussed at great
length, but I will here take only a single case, thatof working or sterile ants. How the workers havebeen rendered sterile is a difficulty; but not muchgreater than that of any other striking modification of
structure; for it can be shown that some insects andother articulate animals in a state ofnature occasionallybecome sterile ; and if such insects had been social, andit had been profitable to the community that a number
should have been annually born capable of work, but incapable of procreation, I can see no very great difficultyin this being effected by natural selection. But I must
pass over this preliminary difficulty. The great difficultylies in the working ants differing widely from both themales and the fertile females in structure, as in the shapeof the thorax and in being destitute of wings and sometimes of eyes, and in instinct. As far as instinct alone
is concerned, the prodigious difference in this respect
between the workers and the perfect females, would
have been far better exemplified by the hive-bee. If aworking ant or other neuter insect had been an animalin the ordinary state, I should have unhesitatingly
assumed that all its characters had been slowly acquired
through natural selection ; namely, by an individual

CHAP. VII.

NEUTER INSECTS.

237

having been born with some slight profitable modification of structure, this being inherited by its offspring,which again varied and were again selected, and so
onwards. But with the working ant we have an insect
differing greatly from its parents, yet absolutely sterile ;so that it could never have transmitted successively
acquired modifications of structure or instinct to its progeny. It may well be asked how is it possible to reconcile this case with the theory of natural selection?
First, let it be remembered that we have innumerable
instances, both in our domestic productions and in thosein a state of nature, of all sorts of differences of structure which have become correlated to certain ages, andto either sex. We have differences correlated not only
to one sex, but to that short period alone when the reproductive system is active, as in the nuptial plumage ofmany birds, and in the hooked jaws of the male salmon.
We have even slight differences in the horns of different
breeds of cattle in relation to an artificially imperfectstate of the male sex ; for oxen of certain breeds have
longer horns than in other breeds, in comparison withthe horns of the bulls or cows of these same breeds.
Hence I can see no real difficulty in any character
havirlg become correlated with the sterile condition ofcertain members of insect-communities : the difficultylies in understanding how such correlated modificationsof structure could have been slowly accumulated bynatural selection.
This difficulty, though appearing insuperable, islessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family,
as well as to the individual, and may thus gain thedesired end. Thus, a well-flavoured vegetable is cooked,
and the individual is destroyed ; but the horticulturist
sows seeds of the same stock, and confidently expects to

238

INSTINCT.

CEAP. VII.

get nearly the same variety ; breeders of cattle wish theflesh and fat to be well marbled together ; the animalhas been slaughtered, but the breeder goes with confidence to the same family. I have such faith in the
powers of selection, that I do not doubt that a breedof cattle, always yielding oxen with extraordinarily longhorns, could be slowly formed by carefully watchingwhich individual bulls and cows, when matched, produced
oxen with the longest horns ; and yet no one ox could ever
have propagated its kind. Thus I believe it has beenwith social insects : a slight modification of structure, or
instinct, correlated with the sterile condition of certain
members of the community, has been advantageous tothe community: consequently the fertile males andfemales of the same community flourished, and transmitted to their fertile offspring a tendency to produce
sterile members having the same modification. And
I believe that this process has been repeated, until thatprodigious amount of difference between the fertile andsterile females of the same species has been produced,
which we see in many social insects.
But we have not as yet touched on the climax of the
difficulty; namely, the fact that the neuters of several
ants differ, not only from the fertile females and males,
but from each other, sometimes to an almost incredible
degree, and are thus divided into two or even threecastes. The castes, moreover, do not generally graduate into each other, but are perfectly well defined ;being as distinct from each other, as are any two speciesof the same genus, or rather as any two genera of thesame family. Thus in Eciton, there are working andsoldier neuters, with jaws and instincts extraordinarily
different : in Cryptocerus, the workers of one caste alone
carry a wonderful sort of shield on their heads, the useof which is quite unknown: in the Mexican Myrme-

CHAP.VII.

NEUTER INSECTS.

239

cocystus, the workers of one caste never leave the nest ;they are fed by the workers of another caste, and they
have an enormously developed abdomen which secretes
a sort of honey, supplying the place of that excreted by
the aphides, or the domestic cattle as they may be called,which our European ants guard or imprison.
It will indeed be thought that I have an overweening
confidence in the principle of natural selection, when I
do not admit that such wonderful and well-establishedfacts at once annihilate my theory. I n the simpler
case of neuter insects all of one caste or of the samekind, which have been rendered by natural selection, as
I believe to be quite possible, different from the fertile
males and females,-in this case, we may safely conclude
from the analogy of ordinary variations, that each successive, slight, profitable modification did not probably
at first appear in all the individual neuters in the same
nest, but in a few alone ; and that by the long-conhued
selection of the fertile parents which produced mostneuters with the profitable modification, all the neuters
ultimately came to have the desired character. On thisview we ought occasionally to find neuter-insects of thesame species, in the same nest, presenting gradations ofstructure ; and this we do find, even often, consideringhow
few neuter-insects out of Europe have been carefully
examined. Mr. F. Smith has shown how surprisinglythe
neuters of several British ants differ from each other insize and sometimes in colour; and that the extreme
forms can sometimes be perfectly linked together by individuals taken out of the same nest: I have myself
compared perfect gradations of this kind. It often
happens that the larger or the smaller sized workers
are the most numerous; or that both large and small
are numerous, with those of an intermediate size
scanty in numbers. Formica flava has larger and240

INSTINCT.

CHAP.VII.

smaller workers, with some of intermediate size ; and,in this species, as Mr. F. Smith has observed, tholarger workers have simple eyes (ocelli), which thoughsmall can be plainly distinguished, whereas the smaller
workers have their ocelli rudimentary. Having carefully dissected several specimens of these workers, I
can affirm that the eyes are far more rudimentary inthe smaller workers than can be accounted for merely
by their proportionally lesser size ; and I fully believe,though I dare not assert so positively, that the workers
of intermediate size have their ocelli in an exactly intermediate condition. So that we here have two bodies
of sterile workers in the same nest, differing not only insize, but in their organs of vision, yet connected by somefew members in an intermediate condition. I may
digress by adding, that if the smaller workers had been
the most useful to the community, and those males andfemales had been continually selected, which produced
more and more of the smaller workers, until all theworkers had come to be in this condition; we should
then have had a species of ant with neuters very
nearly in the same condition with those of Myrmica.
For the workers of Myrmica have not even rudimentsof ocelli, though the male and female ants of this genus
have well-developed ocelli.
I may give one other case: so confidently did I
expect to find gradations in important points of structure
between the different castes of neuters in the same species, that I gladly availed myself of Mr. F. Smith's offer
of numerous specimens from the same nest of the driverant (Anomma) of West Africa. The reader will perhaps best appreciate the amount of difference in theseworkers, by my giving not the actual measurements,
but a strictly accurate illustration: the difference was
the same as if we were to see a set of workmen building

CHAP. VII.

NEUTER INSECTS.

241

a house of whom many were five feet four inches high,and many sixteen feet high ; but we must suppose thatthe larger workmen had heads four instead of threet,imesas big as those of the smaller men, and jaws nearlyfive times as big. The jaws, moreover, of the working
ants of the several sizes differed wonderfully in shape,and in the form and number of the teeth. But theimportant fact for us is, that though the workers can begrouped into castes of different sizes, yet they graduate
insensibly into each other, as does the widely-different
Et,ruCtUre of their jaws. I speak confidently on thislatter point, as Mr. Lubbock made drawings for mewith the camera lucida of the jaws which I had dissected from the workers of the several sizes.
With these facts before me, I believe that natural
selection, by acting on the fertile parents, could forma species whicb should regularly produce neuters,either all of large size with one form of jaw, or all ofsmall size with jaws having a widely different structure ;or lastly, and this is our climax of difficulty, one set ofworkers of one size and structure, and simultaneously
another set of workers of a different size and structure ;-a graduated series having been first formed, as in
the case of the driver ant, and then the extreme forms,
from being the most useful to the community, having
been produced in greater and greater numbers throughthe natural selection of the parents which generated
them ; until none with an intermediate structure were
produced.
Thus, as I believe, the wonderful fact of two distinctly
defined castes of sterile workers existing in the same
nest, both widely different from each other and fromtheir parents, has originated. We can see how usefultheir production may have been to a social communityof insects, on the same principle that the division of

242

INSTINCT.

CHAP. VII.

labour is useful to civilised man. As ants work byinherited instincts and by inherited tools or weapons,
and not by acquired knowledge and manufactured instruments, a perfect division of labour could be effectedwith them only by the workers being sterile; for had
they been fertile, they would have intercrossed, andtheir instincts and structure would have become blended.
And nature has, as I believe, effected this admirable
division of labour in the communities of ants, by themeans of natural selection. But I am bound to confess,
that, with all my faith in this principle, I should never
have anticipated that natural selection could have been
efficient in so high a degree, had not the case of theseneuter insects convinced me of the fact. I have, therefore, discussed this case, at some little but wholly ins&ficient length, in order to show the power of natural
selection, and likewise because this is by far the mostserious special difliculty, which my theory has encountered. The case, also, is very interesting, as it provesthat with animals, as with plants, any amount of modification in structure can be effected by the accumulation of numerous, slight, and as we must call them accidental, variations, which are in any manner profitable,
without exercise or habit having come into play. For
no amount of exercise, or habit, or volition, in the utterly
sterile members of a community could possibly have affected the structure or instincts of the fertile members,
which alone leave descendants. I am surprised that noone has advanced this demonstrative case of neuter insects, against the well-known doctrine of Lamarck.

Summary.-I have endeavoured briefly in this chapter
to show that the mental qualities of our domestic
animals vary, and that the variations are inherited.
Still more briefly I have attempted to show that in-CHAP.VII.

SUMMARY.

243

stincts vary slightly in a state of nature. No one willdispute that instincts are of the highest importance toeach animal. Therefore I can see no difficulty, underchanging conditions of life, in natural selection accumulating slight modifications of instinct to any extent,
in any useful direction. I n some cases habit or use
and disuse have probably come into play. I do notpretend that the facts given in this chapter strengthen
in any great degree my theory ; but none of thn casesof difficulty, to the best of my judgment, annihilate it.
On the other hand, the fact that instincts are not always
absolutely perfect and are liable to mistakes ;-that no
instinct has been produced for the exclusive good ofother animals, but that each animal takes advantage ofthe instincts of others ;-that the canou in natural history, of ‘‘ natura non facit saltum” is applicable to instincts as well as to corporeal structure, and is plainly
explicable on the foregoing views, but is otherwise inexplicable,-all tend to corroborate the theory of natural
selection.
This theory is, also, strengthened by some few otherfacts in regard to instincts ; as by that common case of
closely allied, but certainly distinct, species, when inhabiting distant parts of the world and living underconsiderably different conditions of life, yet often retaining nearly the same instincts. For instance, we canunderstand on the principle of inheritance, how it isthat the thrush of South America lines its nest withmud, in the same peculiar manner as does our British
thrush : how it is that the male wrens (Troglodytes) of
North America, build ‘‘ cock-nests,” to roost in, like themales of our distinct Kitty-wrens,-a habit wholly unlikethat of any other known bird. Finally, it may not be
a logical deduction, but to my imagination it is far
more satisfactory to look at such instincts as the young

244

INSTINCT.

CHAP. VII.

cuckoo ejecting its foster-brothers,-ants making slaves,-the larvae of ichneumonidae feeding within the live
bodies of caterpillars,-notas specially endowed or
created instincts, but as small consequences of one
general law, leading to the advancement of all organic
beings, namely, multiply, vary, let the strongest live
and the weakest die.

CHAP. VIII.

HYRRIDISM.

245

CHAPTER VIII.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids Sterility various in degree, not universal, affected by close interbreeding, removed by domestication-Laws governing the sterilityof hybrids- Sterility not a special endowment, but incidental
on other differences- Causes of the sterility of first crosses andof hybrids - Parallelism between the effects of changed conditions of life and crossing -Fertility of varieties when crossed
and of their mongrel offspring not universal - Hybrids andmongrels compared independently of their fertility - Summary.

THE view generally entertained by naturalists is thatspecies, when intercrossed, have been specially endowedwith the quality of sterility, in order to prevent theconfusion of all organic forms. This view certainly
seems at first probable, for species within the same
country could hardly have kept distinct, had they beencapable of crossing freely. The importance of the factthat hybrids are very generally sterile, has, I think,been much underrated by some late writers. On thetheory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not
possibly be of any advantage to them, and therefore
could not have been acquired by the continued preservation of successive profitable degrees of sterility. I
hope, however, to be able to show that sterility is nota specially acquired or endowed quality, but is incidental on other acquired differences.
I n treating this subject, two classes of facts, to alarge extent fundamentally different, have generally
been confounded together ; namely, the sterility of two

246

HYBRIDISM.

CHAP. VIII.

species when first crossed, and the sterility of the hybrids
produced from them.
Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they
produce either few or no offspring. Hybrids, on theother hand, have their reproductive organs functionallyimpotent, as may be clearly seen in the state of themale element in both plants and animals ; though theorgans themselves are perfect in structure, as far as the
microscope reveals. In the first case the two sexual
elements which go to form the embryo are perfect ; inthe second case they are either not at all developed, or
are imperfectly developed. This distinction is important, when the cause of the sterility, which is common to
the two cases, has to be considered. The distinction has
probably been slurred over, owing to the sterility inboth cases being looked on as a special endowment,
beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known
or believed to have descended from common parents,
when intercrossed, and likewise the fertility of theirmongrel offspring, is, on my theory, of equal importance with the sterility of species; for it seems to
make a broad and clear distinction between varieties
and species.
First, for the sterility of species when crossed and of
their hybrid offspring. It is impossible to study theseveral memoirs and works of those two conscientious and admirable observers, Kolreuter and G a n e r ,who almost devoted their lives to this subject, without
being deeply impressed with the high generality ofsome degree of sterility. Kolreuter makes the rule
universal ; but then he cuts the knot, for in ten cases
in which he found two forms, considered by mostauthors as distinct species, quite fertile together, heCHAP. VIII.

STERILITY.

247

unhesitatingly ranks them as varieties. Gartner, also,
makes the rule equally universal ; and he disputes theentire fertility of Kolreuter’s ten cases. But in theseand in many other cases, Gartner is obliged carefully
to count the seeds, in order to show that there isany degree of sterility. He always compares the
maximum number of seeds produced by two species
when crossed and by their hybrid offspring, with theaverage number produced by both pure parent-species
in a state of nature. But a serious cause of error seems
to me to be here introduced: a plant to be hybridised
must be castrated, and, what is often more important,must be secluded in order to prevent pollen beingbrought to it by insects from other plants. Nearly all theplants experimentised on by Gartner were potted, andapparently were kept in a chamber in his house. That
these processes are often injurious to the fertility of aplant cannot be doubted ; for Girtner gives in his table
about a score of cases of plants which he castrated, andartificially fertilised with their own pollen, and (excluding all cases such m the Leguminosae, in which there
is an acknowledged difficulty in the manipulation) halfof these twenty plants had their fertility in some degree
impaired. Moreover, as Gartner during several years
repeatedly crossed the primrose and cowslip, which wehave such good reason to believe to be varieties,
and only once or twice succeeded in getting fertile
seed; as he found the common red and blue pimpernels (Anagallis arvensis and ccerulea), which thebest botanists rank as varieties, absolutely sterile together; and as he came to the same concluson inseveral other analogous cases ; it seems to me that we
may well be permitted to doubt whether many otherspecies are really so sterile, when intercrossed, as Garb
ner believes.

248

HYBRIDISM.

CHAP. VIII.

It is certain, on the one hand, that the sterility ofvarious species when crossed is so different in degree
and graduates away so insensibly, and, on the other
hand, that the fertility of pure species is so easily
affected by various circumstances, that for all practical
purposes it is most difficult to say where perfect fertility
ends and sterility begins. I think no better evidenceof this can be required than that the two most experienced observers who have ever lived, namely, K6lreuterand Gartner, should have arrived at diametrically opposite conclusions in regard to the very same species.
It is also most instructive to compare-but I have notspace here to enter on details-the evidence advancedby our best botanists on the question whether certain
doubtful forms should be ranked as species or varieties,
with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments
made during different years. It can thus be shown thatneither sterility nor fertility affords any clear distinction
between species and varieties ; but that the evidence
from this source graduates away, and is doubtful in thesame degree as is the evidence derived from other constitutional and structural differences.
I n regard to the sterility of hybrids in successive
generations ; though Gartner was enabled to rear somehybrids, carefully guarding them from a cross witheither pure parent, for six or seven, and in one case forten generations, yet he asserts positively that their fertility never increased, but generally greatly decreased.
I do not doubt that this is usually the case, and thatthe fertility often suddenly decreases in the first few
generations. Nevertheless I believe that in all theseexperiments the fertility has been diminished by anindependent cause, namely, from close interbreeding.
I have collected so large a body of facts, showing

CHAP. VIII.

STERILITY.

249

that close interbreeding lessens fertility, and, on theother hand, that an occasional cross with a distinct individual or variety increases fertility, that I cannot doubt
the correctness of this almost universal belief umongst
breeders. Hybrids are seldom raised by experimentalists in great numbers ; and as the parent-species, orother allied hybrids, generally grow in the same garden,
the visits of insects must be carefully prevented during
the flowering season : hence hybrids will generally be
fertilised during each generation by their own individual pollen; and I am conviced that this would be
injurious to their fertility, already lessened by theirhybrid origin. I am strengthened in this conviction
by a remarkable statement repeatedly made by
Gartner, namely, that if even the less fertile hybrids
be artificially fertilised with hybrid pollen of the mme
bind, their fertility, notwithstanding the frequent ill
effects of manipulation, sometimes decidedly increases,
and goes on increasing. Now, in artificial fertilisation
pollen is as often taken by chance (as I know from my
own experience) fiom the anthers of another flower, as
Gom the anthers of the flower itself which is to be
fertilised; so that a cross between two flowers, thoughprobably on the same plant, would be thus effected.
Moreover, whenever complicated experiments are in
progress, so careful an observer as Gartner would havecastrated his hybrids, and this would have insured ineach generation a cross with the pollen from a distinct
flower, either from the same plant or from another plantof the same hybrid nature. And thus, the strange fact of
the increase of fertility in the successive generationsof artajkially fertiliwd hybrids may, I believe, be accounted for by close interbreeding having been avoided.
Now let us turn to the results arrived at by the third
most experienced hybridiser, namely, the Hen. and250

HYBRIDISM.

CHAP. VIII.

Rev. W. Herbert. H e is as emphatic in his conclusionthat some hybrids are perfectly fertile-as fertile as thepure parent-species-as are Kolreuter and Giirtner thatsome degree of sterility between distinct species is auniversal law of nature. He experimentised on someof the very same species as did Gartner. The difference in their results may, I think, be in part accounted for by Herbert’s great horticultural skill, andby his having hothouses at his command. Of his many
important statements I will here give only a single
one as an example, namely, that “every ovule in a
pod of Crinum capense fertilised by C. revolutum produced a plant, which (he says) I never saw to occur ina case of its natural fecundation.” So that we here
have perfect, or even more than commonly perfect, fertility in a first cross between two distinct species.
This case of the Crinum leads me to refer to a most
singular fact, namely, that there are individual plants,
as with certain species of Lobelia, and with all the
species of the genus Hippeastrum, which can be far moreeasily fertilised by the pollen of another and distinct
species, than by their own pollen. For these plants
have been found to yield seed to the pollen of a distinct
species, though quite sterile with their own pollen, notwithstanding that their own pollen was found to be perfectly good, for it fertilised distinct species. So thatcertain individual plants and all the individuals of certain species can actually be hybridised much more
readily than they can be self-fertilised ! For instance,
a bulb of Hippeastrum aulicum produced four flowers ;three were fertilised by Herbert with their own pollen,
and the fourth was subsequently fertilised by the pollenof a compound hybrid descended from three other anddistinct species: the result was that “the ovaries of
the three first flowers soon ceased to grow, and after aCHAP. VIII.

STERILITY.

251

few days perished entirely, whereas the pod impregnated by the pollen of the hybrid made vigorous
growth and rapid progress to maturity, and bore good
seed, which vegetated freely.” I n a letter to me, in
1839, Mr. Herbert told me that he had then tried theexperiment during five years, and he continued to t,ry
it during several subsequent years, and always with thesame result. This result has, also, been confirmed byother observers in the case of Hippeastrum with its
subgenera, and in the case of some other genera, as
Lobelia, Passiflora and Verbascum. Although the plants
in these experiments appeared perfectly healthy, andalthough both the ovules and pollen of the same
flower were perfectly good with respect to other species,
yet as they were functionally imperfect in their mutual
self-action, we must infer that the plants were in an
unnatural state. Nevertheless these facts show on whatslight and mysterious causes the lesser or greater fertility of species when crossed, in comparison with thesame species when self-fertilised, sometimes depends.
The practical experiments of horticulturists, though
not made with scientific precision, deserve somenotice. It is notorious in how complicated a manner
the species of Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, &c., have been crossed, yet manyof these hybrids seed freely. For instance, Herbertasserts that a hybrid from Calceolaria integrifolis
and plantaginea, species most widely dissimilar ingeneral habit, “reproduced itself as perfectly as if
it had been a natural species from the mountains ofChile.” I have taken some pains to ascertain the
degree of fertility of some of the complex crosses of
Rhododendrons, and I am assured that many of themare perfectly fertile. Mr. C. Noble, for instance, informs
me that he raises stocks for grafting from a hybrid

252

HYBRIDISY.

CHAP. VIII.

between Rhod. Ponticum and Catawbiense, and that
this hybrid “seeds as freely as it is possible to imagine.” Had hybrids, when fairly treated, gone on decreasing in fertility in each successive generation, aa
Giirtner believes to be the case, the fact would havebeen notorious to nurserymen. Horticulturists raiselarge beds of the same hybrids, and such alone are
fairly treated, for by insect agency the several individuals of the same hybrid variety are allowed to freely
cross with each other, and the injurious influence of
close interbreeding is thus prevented. Any one may
readily convince himself of the efficiency of insectagency by examining the flowers of the more sterilekinds of hybrid rhododendrons, which produce no pollen, for he will find on their stigmas plenty of pollen
brought from other flowers.
I n regard to animals, much fewer experiments havebeen carefully tried than with plants. If our systematicarrangements can be trusted, that is if the genera ofanimals are as distinct from each other, as are the generaof plants, then we may infer that animals more widely
separated in the scale of nature can be more easily
crossed than in the case of plants; but the hybrids
themselves are, I think, more sterile. I doubt whether
any case of a perfectly fertile hybrid animal can beconsidered as thoroughly well authenticated. It should,
however, be borne in mind that, owing to few animals
breeding freely under confinement, few experiments
have been fairly tried: for instance, the canary-bird
has been crossed with nine other finches, but as not
one of these nine species breeds freely in confinement,
we have no right to expect that the h t crosses between them and the canary, or that their hybrids,
should be perfectly fertile. Again, with respect to thefertility in successive generations of the more fertileC H U . I7III.

STERILITY.

253

hybrid animals, I hardly know of an instance in which
two families of the same hybrid have been raised at thesame time from different parents, so as to avoid the ill
effects of close interbreeding. On the contrary, brothers and sisters have usually been crossed in each successive generation, in opposition to the constantly repeated admonition of every breeder. And in this case,
it is not at all surprising that the inherent sterility inthe hybrids should have gone on increasing. If wewere to act thus, and pair brothers and sisters in thecase of any pure animal, which from any cause had
the least tendency to sterility, the breed would assuredly be lost in a very few generations.
Although I do not know of any thoroughly well-authenticated cases of perfectly fertile hybrid animals, I havesome reason to believe that the hybrids from Cervulus
vaginalis and Reevesii, and from Phasianus colchicus
with P. torquatus and with P. versicolor are perfectly
fertile. The hybrids from the common and Chinese
geese (A. cygnoides), species which are so different thatthey are generally ranked in distinct genera, have often
bred in this country with either pure parent, and in
one single instance they have bred inter se. This was
effected by Mr. Eyton, who raised two hybrids from thesame parents but from different hatches; and fromthese two birds he raised no less than eight hybrids(grandchildren of the pure geese) from one nest. I n
India, however, these cross-bred geese must be far
more fertile ; for I am assured by two eminently
capable judges, namely Mr. Blyth and Capt. Hutton,that whole flocks of these crossed geese are kept invarious parts of the country ; and as they are kept for
profit, where neither pure parent-species exists, theymust certainly be highly fertile.
A doctrine which originated with Pallas, has been

254

HYBRIDISM.

CHAP. VIII.

largely accepted by modern naturalists ; namely, thatmost of our domestic animals have descended from two
or more aboriginal species, since commingled by intercrossing. On this view, the aboriginal species must
either at first have produced quite fertile hybrids, orthe hybrids must have become in subsequent generations quite fertile under domestication. This latter
alternative seems to me the most probable, and I am
inclined to believe in its truth, although it rests on no
direct evidence. I believe, for instance, that our dogs
have descended from several wild stocks ; yet, with perhaps the exception of certain indigenous domestic dogsof South America, all are quite fertile together; andanalogy makes me greatly doubt, whether the several
aboriginal species would at first have freely bred together and have produced quite fertile hybrids. So
again there is reason to believe that our European andthe humped Indian cattle are quite fertile together;but from facts communicated to me by Mr. Blyth, I
think they must be considered as distinct species. On
this view of the origin of many of our domestic animals,
we must either give up the belief of the almost universal sterility of distinct species of animals whencrossed; or we must look at sterility, not as an indelible characteristic, but as one capable of being removed by domestication.
Finally, looking to id1 the ascertained facts on theintercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses
and in hybrids, is an extremely general result ; but thatit cannot, under our present state of knowledge, be considered as absolutely universal.

Laws governing the Sterility of Jirst Crosses and of Hybrids.-We will now consider a little more in detail theCHAP. VIII.

LAWS O F STERILITY.

255

circumstances and rules governing the sterility of firstcrosses and of hybrids. Our chief object will be to see
whether or not the rules indicate that species havespecially been endowed with this quality, in order toprevent their crossing and blending together in utter
confusion. The following rules and conclusions arechiefly drawn up from G’artner’s admirable work on the
hybridisation of plants. I have taken much pains toascertain how far the rules apply to animals, and considering how scanty our knowledge is in. regard to hybrid animals, I have been surprised to find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduatesfrom zero to perfect fertility. It is surprising in how
many curious ways this gradation can be shown toexist; but only the barest outline of the facts can here
be given. When pollen from a plant of one family isplaced on the stigma of a plant of a distinct family, it
exerts no more influence than so much inorganic dust.
From this absolute zero of fertility, the pollen of different species of the same genus applied to the stigma ofsome one species, yields a perfect gradation in thenumber of seeds produced, up to nearly complete oreven quite complete fertility; and, as we have seen,
in certain abnormal cases, even to an excess of fertility,
beyond that which the plant’s own pollen will produce.
So in hybrids themselves, there are some which never
have produced, and probably never would produce, eb-enwith the pollen of either pure parent, a single fertile seed :but in some of these cases a fist trace of fertility may
be detected, by the pollen of one of the pure parentspecies causing the flower of the hybrid to wither
earlier than it otherwise would have done; and theearly withering of the flower is well known to be a sign256

HYBRIDISM.

CHAP. VIII.

of incipient fertilisation. From this extreme degreeof sterility we have self-fertilised hybrids producing agreater and greater number of seeds up to perfect fertility.
Hybrids from two species which are very difficult tocross, and which rarely produce any offspring, are generally very sterile ; but the parallelism between the difficulty of making a first cross, and the sterility of thehybrids thus produced-two classes of facts which are
generally confounded together-is by no means strict.
There are many cases, in which two pure species can be
united with unusual facility, and produce numerous
hybrid-offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which
can be crossed very rarely, or with extreme difficulty,
but the hybrids, when at last produced, are very fertile.
Even within the limits of the same genus, for instancein Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, ismore easily affected by unfavourable conditions, thanis the fertility of pure species. But the degree of
fertility is likewise innately variable ; for it is not always
the same when the same two species are crossed under
the same circumstances, but depends in part upon theconstitution of the individuals which happen to havebeen chosen for the experiment. So it is with hybrids,
for their degree of fertility is often found to differ
greatly in the several individuals raised from seed outof the same capsule and exposed to exactly the same
conditions.
By the term systematic affinity is meant, the resemblance between species in structure and in constitution,
more especially in the structure of parts which are ofhigh physiological importance and which differ little in
the allied species. Now the fertility of first crossesCHAP. VIII.

LAWS OF STERILITY.

257

between species, and of the hybrids produced fromthem, is largely governed by their systematic affinity. This is clearly shown by hybrids never having
been raised between species ranked by systematists indistinct families ; and on the other hand, by very closely
allied species generally uniting with facility. But thecorrespondence between systematic affinity and thefacility of crossing is by no means strict. A multitude of cases could be given of very closely allied
species vhich will not unite, or only with extreme
difficulty; and on the other hand of very distinct
species which unite with the utmost facility. I n thesame family there may be a genus, as Dianthus, inwhich very many species can most readily be crossed ;and another genus, as Silene, in which the mostpersevering efforts have failed to produce between
extremely close species a single hybrid. Even within
the limits of the same genus, we meet with this samedifference ; for instance, the many species of Ncotiana
have been more largely crossed than the species ofalmost any other genus; but Gartner found that N.
acuminata, which is not a particularly distinct species,
obstinately failed to fertilise, or to be fertilised by, noless than eight other species of Nicotiana. Very manyanalogous facts could be given.
No one has been able to point out what kind, or whatamount, of difference in any recognisable character issufficient to prevent two species crossing. It can beshown that plants most widely different in habit andgeneral appearance, and having strongly marked differences in every part of the flower, even in the pollen, in
the fruit, and in the cotyledons, can be crossed. Annual
and perennial plants, deciduous and evergreen trees,
plants inhabiting different stations and fitted for extremely different climates, can often be crossed with ease.

258

HYBRIDISM.

CHAP. VIII.

By a reciprocal cross between two species, I mean
the case, for instance, of a stallion-horse being firstcrossed with a female-ass, and then a male-ass with amare : these two species may then be said to have been
reciprocally crossed. There is often the widest possible
difference in the facility of making reciprocal crosses.
Such cases are highly important, for t,hey prove thatthe capacity in any two species to cross is often completely independent of their systematic aEnity, or of
any recognisable difference in their whole organisation.
On the other hand, these cases clearly show that thecapacity for crossing is connected with constitutional
differences imperceptible by us, and confined to thereproductive system. This difference in the result ofreciprocal crosses between the same two species waslong ago observed by Kolreuter. To give an instance :Mirabilis jalappa can easily be fertilised by the pollenof M. longiflora, and the hybrids thus produced are
sufficiently fertile ; but Kolreuter tried more than two
hundred times, during eight following years, to fertilise
reciprocally M. longiflora with the pollen of M. jalappa,
and utterly failed. Several other equally striking cases
could be given. Thuret has observed the same factwith certain sea-weeds or Fuci. Gartner, moreover,found that this difference of facility in making reciprocal crosses is extremely common in a lesser degree.
H e has observed it even between forms so closely related(as Matthiola annua and glabra) that many botanists
rank them only as varieties. It is also a remarkablefact, that hybrids raised from reciprocal crosses, thoughof course compounded of the very same two species,
the one species having first been used as the father andthen as the mother, generally differ in fertility in asmall, and occasionally in a high degree.
Several other singular rules could be given fromCHAP. VIII.

LAWS O F STERILITY.

259

Gartner : for instance, some species have a remarkablepower of crossing with other species; other species ofthe same genus have a remarkable power of impressing
their likeness on their hybrid offspring; but thesetwo powers do not at all necessarily go together.
There are certain hybrids which instead of having, as
is usual, an intermediate character between their twoparents, always closely resemble one of them; andsuch hybrids, though externally so like one of their pureparent-species, are with rare exceptions extremely
sterile. So again amongst hybrids which are usuallyintermediate in structure between their parents, exceptional and abnormal individuals sometimes are born,which closely resemble one of their pure parents ; andthese hybrids are almost always utterly sterile, evenwhen the other hybrids raised from seed from the same
capsule have a considerable degree of fertility. These
facts show how completely fertility in the hybrid isindependent of its external resemblance to either pureparent.
Considering the several rules now given, whichgovern the fertility of first crosses and of hybrids, we
see that when forms, which must be considered as
good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to
fertility under certain conditions in excess. That
their fertility, besides being eminently susceptible tofavourable and unfavourable conditions, is innately
variable. That it is by no means always the same indegree in the first cross and in the hybrids produced
from this cross. That the fertility of hybrids is not
related to the degree in which they resemble in external appearance either parent. And lastly, that thefacility of making a first cross between any two species
is not always governed by their systematic affinity or

260

HYBRIDISM,

CHAP. VIII.

degree of resemblance to each other. This latterstatement is clearly proved by reciprocal crosses
between the same two species, for according as theone species or the other is used as the father orthe mother, there is generally some difference, andoccasionally the widest possible difference, in thefacility of effecting an union. The hybrids, moreover, produced from reciprocal crosses often differ in
fertility.
Now do these complex and singular rules indicatethat species have been endowed with sterility simply
to prevent their becoming confounded in nature? I
think not. For why should the sterility be so extremely
different in degree, when various species are crossed, allof which we must suppose it would be equally importantto keep from blending together? Why should thedegree of sterility be innately variable in the individualsof the same species ? Why should some species crosswith facility, and yet produce very sterile hybrids;
and other species cross with extreme difficulty, and yet
produce fairly fertile hybrids ? Why should there often
be so great a difference in the result of a reciprocal
cross between the same two species? Why, it may
even be asked, has the production of hybrids been permitted? to grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related
to the facility of the first union between their parents,
seems to be a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility both offirst crosses and of hybrids is simply incidental or dependent on unknown differences, chiefly in the reproductive systems, of the species which are crossed. The
differences being of so peculiar and limited a nature,

CHAP. VIII.

COMPARED WITH GRAFTING.

261

that, in reciprocal crosses between two species the malesexual element of the one will often freely act on thefemale sexual element of the other, but not in a reversed direction. It will be advisable to explain a little
more fully by an example what I mean by sterility
being incidental on other differences, and not a specially endowed quality. As the capacity of one plant tobe grafted or budded on another is so entirely unimportant for its welfare in a state of nature, I presume
that no one will suppose that this capacity is a 8pe'eciallyendowed quality, but will admit that it is incidenial
on differences in the laws of growth of the two plants.
We can sometimes see the reason why one tree will
not take on another, from differences in their rate of
growth, in the hardness of their wood, in the period of
the flow or nature of their sap, &c. ; but in a multitude
of cases we can assign no reason whatever. Great diversity in the size of two plants, one being woody andthe other herbaceous, one being evergreen and theother deciduous, and adaptation to widely different
climates, does not always prevent the two grafting together. As in hybridisation, so with grafting, thecapacity is limited by systematic affinity, for no one
has been able to graft trees together belonging to quitedistinct families ; and, on the other hand, closely allied
species, and varieties of the same species, can usually,
but not invariably, be grafted with ease. But this capacity, as in hybridisation, is by no means absolutely governed by systematic affinity. Although many distinct
genera within the same family have been grafted together, in other cases species of the same genus will
not take on each other. The pear can be graftedfar
more readily on the quince, which is ranked as a distinct
genus, than on the apple, which is a member of thesame genus. Even different varieties of the pear take

262

HYBRIDISM.

CHAP. 17111.

with different degrees of facility on the quince ; so dodifferent varieties of the apricot and peach on certain
varieties of the plum.
As Gartner found that there was sometimes an innate
difference in different individuals of the same two species in crossing ; so Sagaret believes this to be the casewith different individuals of the same two species inbeing grafted together. As in reciprocal crosses, thefacility of effecting an union is often very far fromequal, so it sometimes is in grafting; the common
gooseberry, for instance, cannot be grafted on the currant, whereas the currant will take, though with difficulty, on the gooseberry.
We have seen that the sterility of hybrids, whichhave their reproductive organs in an imperfect condition, is a very different case from the difficulty of
uniting two pure species, which have their reproductive organs perfect; yet these two distinct cases run
to a certain extent parallel. Something analogous
occurs in grafting ; for Thouin found that three speciesof Robinia, which seeded freely on their own roots,
and which could be grafted with no great difficulty
on another species, when thus grafted were renderedbarren. On the other hand, certain species of Sorbus,
when grafted on other species, yielded twice as muchfruit as when on their own roots. We are reminded bythis latter fact of the extraordinary case of Hippeastrum, Lobelia, &c., which seeded much more freely
when fertilised with the pollen of distinct species, thanwhen self-fertilised with their own pollen.
We thus see, that although there is a clear andfundamental difference between the mere adhesion ofgrafted stocks, and the union of the male and female
elements in the act of reproduction, yet t,hat there is arude degree of parallelism in the results of grafting andCHAP. VIII.

CAUSES OF STERILITY.

263

of crossing distinct species. And as we must look at
the curious and complex laws governing the facility withwhich trees can be grafted on each other as incidental
on unknown differences in their vegetative systems, so
I believe that the still more complex laws governing
the facility of first crosses, are incidental on unknown
differences, chiefly in their reproductive systems.
These differences, in both cases, follow to a certain
extent, as might have been expected, systematic affinity,by which every kind of resemblance and dissimilarity
between organic beings is attempted to be expressed.
The facts by no means seem to me to indicate that thegreater or lesser d%culty of either grafting or crossing
together various species has been a special endowment; although in the case of crossing, the difficultyis as important for the endurance and stability of specific forms, as in the case of grafting it is unimportant
for their welfare.

Causes of the Sterility of first Crosses and of Ry6n'ds.We may now look a little closer at the probable causesof the sterility of h t crosses and of hybrids. These
two cases are fundamentally different, for, as justremarked, in the union of two pure species the maleand female sexual elements are perfect, whereas in
hybrids they are imperfect. Even in first crosses, thegreater or lesser difficulty in effecting a union apparently
depends on several distinct causes. There must sometimes be a physical impossibility in the male element
reaching the ovule, as would be the case with a plant
having a pistil too long for the pollen-tubes to reach theovarium. It has also been observed that when pollenof one species is placed on the stigma of a distantly
allied species, though the pollen-tubes protrude, they
do not penetrate the stigmatic aurface. Again, the264

HYBRIDISM.

CHAP. VIII.

male element may reach the female element, but beincapable of causing an embryo to be developed, as
seems to have been the case with some of Thuret’s
experiments on Fuci. No explanation can be givenof these facts, any more than why certain trees cannot
be grafted on others. Lastly, an embryo may be
developed, and then perish at an early period. Thislatter alternative has not been sufficiently attended to ;but I believe, from observations communicated to me by
Mr. Hewitt, who has had great experience in hybridisinggallinaceous birds, that the early death of the embryo
is a very frequent cause of sterility in first crosses. I
was at first very unwilling to believe in this view;as hybrids, when once born, are generally healthy
and long-lived, as we see in the case of the common
mule. Hybrids, however, are differently circumstanced
before and after birth : when born and living in a country where their two parents can live, they are generally placed under suitable conditions of life. But a
hybrid partakes of only half of the nature and constitution of its mother, and therefore before birth, m longas it is nourished within its mother’s womb or within
the egg or seed produced by the mother, it may be
exposed to conditions in some degree unsuitable,
and consequently be liable to perish at an early
period ; more especially as all very young beings seememinently sensitive to injurious or unnatural conditions of life.
I n regard to the sterility of hybrids, in which thesexual elements are imperfectly developed, the case isvery different. I have more than once alluded to alarge body of facts, which I have collected, showingthat when animals and plants are removed from theirnatural conditions, they are extremely liable to have theirreproductive systems seriously affected. This,in fact, isCHAP. TIII.

CAUSES OF STERILITY.

265

the great bar to the domestication of animals. Between
the sterility thus superinduced and that of hybrids, there
are many points of similarity. I n both cases the sterility
is independent of general health, and is often accompanied by excess of size or great luxuriance. I n bothcases, the sterility occurs in various degrees; in both,the male element is the most liable to be affected ; but
sometimes the female more than the male. I n both,the tendency goes to a certain extent with systematicaffinity, for whole groups of animals and plants are rendered impotent by the same unnatural conditions ; andwhole groups of species tend to produce sterile hybrids.
On the other hand, one species in a group will sometimes resist great changes of conditions with unimpaired
fertility; and certain species in a group will produce
unusually fertile hybrids. No one can tell, till he tries,
whether any particular animal will breed under confinement or any plant seed freely under culture; nor can
he tell, till he tries, whether any two species of a genus
will produce more or less sterile hybrids. Lastly, whenorganic beings are placed during several generations
under conditions not natural to them, they are extremely liable to vary, which is due, as I believe,to their reproductive systems having been speciallyaffected, though in a lesser degree than when sterility
ensues, So it is with hybrids, for hybrids in successive
generations are eminently liable to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed
under new and unnatural conditions, and when hybrids
are produced by the unnatural crossing of two species,
the reproductive system, independently of the generalstate of health, is affected by sterility in a very similarmanner. I n the one case, the conditions of life havebeen disturbed, though often in so slight a degree as to266

HYBRIDISM.

CHAP.VIII.

be inappreciable by us; in the other case, or that ofhybrids, the external conditions have remained the same,but the organisation has been disturbed by two different
structures and constitutions having been blended into
one. For it is scarcely possible that two organisations
should be compounded into one, without some disturbance occurring in the development, or periodical action,or mutual relation of the different parts and organs one
to another, or to the conditions of life. When hybrids
are able to breed inter 86, they transmit to their offspringfrom generation to generation the same compoundedorganisation, and hence we need not be surprised that
their sterility, though in some degree variable, rarely
diminishes.
It must, however, be confessed that we cannot understand, excepting on vague hypotheses, several facts withrespect to the sterility of hybrids; for instance, theunequal fertility of hybrids produced from reciprocal
crosses ; or the increased sterility in those hybrids whichoccasionally and exceptionally resemble closely either
pure parent. Nor do I pretend that the foregoingremarks go to the root of the matter: no explanation
is offered why an organism, when placed under unnatural conditions, is rendered sterile. All that I haveattempted to show, is that in two cases, in some respects
allied, sterility is the common result,-in the one casefrom the conditions of life having been disturbed, in theother case from the organisation having been disturbed
by two organisations having been compounded into one.
It may seem fanciful, but I suspect that a similar
parallelism extends to an allied yet very different classof facts. It is an old and almost universal belief,
founded, I tliink, on a considerable body of evidence,that slight changes in the conditions of life are beneficial to all living things. We see this acted on by

CHAP.VIII.

FERTILITY OF MONGRELS.

267

farmers and gardeners in their frequent exchanges ofseed, tubers, &c., from one soil or climate to another,and back again. During the convalescence of animals,
we plainly see that great benefit is derived from almost
any change in the habits of life. Again, both withplants and animals, there is abundant evidence, that across between very distinct individuals of the same species, that is between members of different strains or
sub-breeds, gives vigour and fertility to the offspring.
I believe, indeed, from the facts alluded to in our fourth
chapter, that a certain amount of crossing is indispensable even with hermaphrodites ; and that close interbreeding continued during several generations between
the nearest relations, especially if these be kept under
the same conditions of life, always induces weakness
and sterility in the progeny.
Hence it seems that, on the one hand, slight changes
in the conditions of life benefit all organic beings, andon the other hand, that slight crosses, that is crosses
between the males and females of the same species
which have varied and become slightly different, give
vigour and fertility to the offspring. But we hare
seen that greater changes, or changes of a particularnature, often render organic beings in some degreesterile ; and that greater crosses, that is crosses between
males and females which have become widely or specifically different, produce hybrids which are generally
sterile in some degree. I cannot persuade myself that
this parallelism is an accident or an illusion. Bothseries of facts seem to be connected together by somecommon but unknown bond, which is essentially related
to the principle of life.

Pertility of Varieties when crossed, and of their Mongrelo#spring.-It may be urged, as a most forcible argu-

268

HYBRIDISY.

CHAP. V11I.

ment, that there must be some essential distinction
between species and varieties, and that there must be
some error in all the foregoing remarks, inasmuch as
varieties, however much they may differ from eachother in external appearance, cross with perfect facility,and yield perfectly fertile offspring. I fully admit that
this is almost invariably the case. But if we look tovarieties produced under nature, we are immediately
involved in hopeless difficulties; for if two hitherto
reputed varieties be found in any degree sterile together, they are at once ranked by most naturalists
as species. For instance, the blue and red pimpernel,
the primrose and cowslip, which are considered bymany of our best botanists as varieties, are said byGartner not to be quite fertile when crossed, and heconsequently ranks them as undoubted species. If wethus argue in a circle, the fertility of all varieties produced under nature will assuredly have to be granted.
If we turn to varieties, produced, or supposed to havebeen produced, under domestication, we are still involved in doubt. For when it is stated, for instance,that the German Spitz dog unites more easily thanother dogs w’th foxes, or that certain South American
indigenous domestic dogs do not readily cross with European dogs, the explanation which will occur to every
one, and probably the true one, is that these dogs havedescended from several aboriginally distinct species.
Nevertheless the perfect fertility of so many domestic
varieties, differing widely from each other in appearance, for instance of the pigeon or of the cabbage, isa remarkable fact; more especially when we reflect
how many species there are, which, though resembling each other most closely, are utterly sterile whenintercrossed. Several considerations, however, render
the fertility of domestic varieties less remarkable than

CHAP. VIII.

FERTILITY OF MONGRELS.

269

at first appears. It can, in the first place, be clearlyshown that mere external dissimilarity between two species does not determine their greater or lesser degree ofsterility when crossed ; and we may apply the same rule
to domestic varieties. I n the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive
generations of hybrids, which were at first only slightly
sterile ; and if this be so, we surely ought not to expect
to find steriIity both appearing and disappearing undernearly the same conditions of life. Lastly, and thisseems to me by far the most important consideration,
new races of animals and plants are produced underdomestication by man’s methodical and unconscious
power of selection, for his own use and pleasure: heneither wishes to select, nor could select: slight differences in the reproductive system, or other constitutional
differences correlated with the reproductive system.
H e supplies his several varieties with the same food;treats them in nearly the same manner, and does notwish to alter their general habits of life. Nature acts
uniformly and slowly during vast periods of time on thewhole organisation, in any way which may be for eachcreature’s own good ; and thus she may, either directly,
or more probably indirectly, through correlation, modify
the reproductive system in the several descendants fromany one species. Seeing this difference in the processof selection, as carried on by man and nature, we neednot be surprised at some difference in the result.
I have as yet spoken as if the varieties of the same
species were invariably fertile when intercrossed. But
it seems to me impossible to resist the evidence of theexistence of a certain amount of sterility in the few
following cases, which I will briefly abstract. The evidence is at least as good as that from which we believe

270

HYBRIDISM.

CHAP.VJII.

in the sterility of a multitude of species. The evidence
is, also, derived from hostile witnesses, who in all othercases consider fertility and sterility as safe criterionsof specific distinction. Gaitner kept during several
years a dwarf kind of maize with yellow seeds, and atall variety with red seeds, growing near each other in
his garden ; and although these plants have separated
sexes, they never naturally crossed. He then fertilised
thirteen flowers of the one with the pollen of the other ;but only a single head produced any seed, and this onehead produced only five grains. Manipulation in thiscase could not have been injurious, as the plants haveseparated sexes. No one, I believe, has suspected thatthese varieties of maize are distinct species; andit is important to notice that the hybrid plants thus
raised were themselves perfectly fertile ; so that even
Gartner did not venture to consider the two varieties as
specifically distinct.
Girou de Buzareingues crossed three varieties ofgourd, which like the maize has separated sexes, andhe asserts that their mutual fertilisation is by so muchthe less easy as their differences are greater. How far
these experiments may be trusted, I know not ; but theforms experimentised on, are ranked by Sagaret, who
mainly founds his classification by the test of infertility,
as varieties.
The following case is far more remarkable, and seems
at first quite incredible ; but it is the result of an astonishing number of experiments made during many years
on nine species of Verbascum, by so good an observerand EO hostile a witness, as Gartner : namely, that yellow
and white varieties of the same species of Verbascum
when intercrossed produce less seed, than do either
coloured varieties when fertilised with pollen from theirown coloured flowers. Moreover, he asserts that when

CHAP.VlII.

FERTILITY OF MONGRELS.

271

yellow and white varieties of one species are crossedwith yellow and white varieties of a distinct species,
more seed is produced by the crosses between the same
coloured flowers, than between those which are differently coloured. Yet these varieties of Verbascum present no ot.her difference besides the mere colour of theflower ; and one variety can sometimes be raised fromthe seed of the other.
From observations which I have made on certain
varieties of hollyhock, I am inclined to suspect thatthey present analogous facts.
Kolreuter, whose accuracy has been confirmed byevery subsequent observer, has proved the remarkablefact, that one variety of the common tobacco is more
fertile, when crossed with a widely distinct species,
than are the other varieties. He experimentised on fiveforms, which are commonly reputed to be varieties, andwhich he tested by the severest trial, namely, by reciprocal crosses, and he found their mongrel offspringperfectly fertile. But one of these five varieties, whenused either as father or mother, and crossed with the
Nicotiana glutinosa, always yielded hybrids not sosterile as those which were produced from the fourother varieties when crossed with N. glutinosa. Hencethe reproductive system of this one variety must havebeen in some manner and in some degree modified.
From these facts ; from the great difficulty of ascertaining the infertility of varieties in a state of nature,
for a supposed variety if infertile in any degree would
generally be ranked as species ; from man selecting only
external characters in the production of the most distinct domestic varieties, and from not wishing or being
able to produce recondite and functional differences in
the reproductive system ; from these several considerations and facts, I do not think that the very general

272

HYBRIDISM.

CHAP.VJII.

fertility of varieties can be proved to be of universal
occurrence, or to form a fundamental distinction between
varieties and species. The general fertility of varieties
does not seem to me sufficient to overthrow the viewwhich I have taken with respect to the very general,
but not invariable, sterility of first crosses and of hybrids,
namely, that it is not a special endowment, but is incidental on slowly acquired modifications, more especiallyin the reproductive systems of the forms which arecrossed.
Hy6rid8 and Mongrels compared, independently of theirf erti1ity.-Independently of t.lie question of fertility, theoffspring of species when crossed and of varieties whencrossed may be compared in several other respects.
Gartner, whose strong wish was to draw a marked lineof distinction between species and varieties, could findvery few and, as it seems to me, quite unimportantdifferences between the so-called hybrid offspring ofspecies, and the so-called mongrel offspring of varieties.
And, on the other hand, they agree most closely in very
many important respects.
I shall here discuss this subject with extreme brevity.
The most important distinction is, that in the first
generation mongrels are more variable than hybrids ;but Gartner admits that hybrids from species whichhave long been cultivated are often variable in the first
generation ; and I have myself seen striking instancesof this fact. Gartner further admits that hybrids between very closely allied species are more variable
than those from very distinct species; and this showsthat the difference in the degree of variability graduates away. When mongrels and the more fertile
hybrids are propagated for several generations an extreme amount of variability in their offspring is notori-

CHAP. 1'111.

HYBRIDS AND MONGRELS.

273

ous ; but some few cases both of hybrids and mongrels
long retaining uniformity of character could be given.
The variability, however, in the successive generationsof mongrels is, perhaps, greater than in hybrids.
This greater variability of mongrels than of hybrids
does not seem to me at all surprising. For the parents
of mongrels are varieties, and mostly domestic varieties(very few experiments having been tried on naturalvarieties), and this implies in most cases that there has
been recent variability ; and therefore we might expectthat such variability would often continue and be sugeradded to that arising from the mere act of crossing.
The slight degree of variability in hybrids from the firstcross or in the first generation, in contrast with theirextreme variability in the succeeding generations, is acurious fact and deserves attention. For it bears on
and corroborates the view which I have taken on thecause of ordinary variability; namely, that it is dueto the reproductive system being eminently sensitive
to any change in the conditions of life, being thus
often rendered either impotent or at least incapableof its proper function of producing ofXspring identicalwith the parent-form, Now hybrids in the first generation are descended from species (excluding thoselong cultivated) which have not had their reproductive systems in any way affected, and they are
not variable ; but hybrids themselves have their reproductive systems seriously affected, and their descendants are highly variable.
But to return to our comparison of mongrels andhybrids : GLtner states that mongrels are more liable
than hybrids to revert to either parent-form ; but
this, if it be true, is certainly only a difference indegree. Gartner further insists that when any two
species, although most closely allied to each other, are274

HYBRIDISM.

CHAP. VIII.

crossed with a third species, the hybrids are widelydifferent from each other ; whereas if two very distinct
varieties of one species are crossed with another species,
the hybrids do not differ much. But this conclusion,as far as I can make out, is founded on a single experiment; and seems directly opposed to the results ofseveral experiments made by Kolreuter.
These alone are the unimportant differences, which
G6rtner is able to point out, between hybrid andmongrel plants. On the other hand, the resemblancein mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species, follows according to Gartner the samelaws. When two species are crossed, one has sometimes a prepotent power of impressing its likenem
on the hybrid ; and so I believe it to be with varietiesof plants. With animals one variety certainly often has
this prepotent power over another variety. Hybrid
plants produced from a reciprocal cross, generally resemble each other closely; and so it is with mongrels
from a reciprocal cross. Both hybrids and mongrels
can be reduced to either pure parentiform, by repeatedcrosses in successive generations with either parent.
These several remarks are apparently applicable toanimals; but the subject is here excessively complicated, partly owing to the existence of secondary sexual
characters ; but more especially owing to prepotency
in transmitting likeness running more strongly in one
sex than in the other, both when one species is crossedwith another, and when one variety is crossed with another variety. For instance, I think those authors areright, who maintain that the ass has a prepotent powerover the horse, so that both the mule and the hinnymore resemble the ass than the horse; but that the
prepotency runs more strongly in the male-ass than inC U P . VIII.

HYBRIDS AND MONGRELS.

275

the female, so that the mule, which is the offspring of
the male-ass and mare, is more like an ass, than is thehinny, which is the offspring of the female-ass andstallion.
Much stress has been laid by some authors on the
supposed fact, that mongrel animals alone are bornclosely like one of their parents ; but it can be shownthat this does sometimes occur with hybrids; yet I
grant much less frequently with hybrids than withmongrels. Looking to the cases which I have collectedof cross-bred animals closely resembling one parent,
the resemblances seem chiefly confined to charactersalmost monstrous in their nature, and which have suddenly appeared-such as albinism, melanism, deficiencyof tail or horns, or additional fingers and toes ; and do
not relate to characters which have been slowly acquiredby selection. Consequently, sudden reversions to theperfect character of either parent would be more likely
to occur with mongrels, which are descended from varieties often suddenly produced and semi-monstrous incharacter, than with hybrids, which are descended from
species slowly and naturally produced. On the whole
I entirely agree with Dr. Prosper Lucas, who, after
arranging an enormous body of facts with respect toanimals, comes to the conclusion, that the laws of resemblance of the child to its parents are the same, whether
the two parents differ much or little from each other,namely in the union of individuals of the same variety,or of different varieties, or of distinct species.
Laying aside the question of fertility and sterility,
in all other respects there seems to be a general andclose similarity in the offspring of crossed species, and ofcrossed varieties. If we look at species as having been
specially created, and at varieties as having been produced by secondary laws, this similarity would be an

276

HYBRIDISM.

CHAP.VIII.

astonishing fact. But it harmonises perfectly with theview that there is no essential distinction between species and varieties.
Summary of Chapter.-First crosses between forms
sufficiently distinct to be ranked as species, and theirhybrids, are very generally, but not universally, sterile.
The sterility is of all degrees, and is often so slight thatthe two most careful experimentalists who have everlived, have come to diametrically opposite conclusions
in ranking forms by this test. The sterility is innately
variable in individuals of the same species, and iseminently susceptible of favourable and unfavourable
conditions. The degree of sterility does not strictly
follow systematic affinity, but is governed by several
curious and complex laws. It is generally different,
and sometimes widely different, in reciprocal crosses
between the same two species. It is not always equal
in degree in a first cross and in the hybrid produced
from this cross.
I n the same manner as in grafting trees, the capacityof one species or variety to take on another, is incidental
on generally unknown differences in their vegetative
systems, so in crossing, the greater or less facility of one
species to unite with another, is incidental on unknown
differences in their reproductive systems. There is no
more reason to think that species have been speciallyendowed with various degrees of sterility to prevent
them crossing and blending in nature, than to thinkthat trees have been specially endowed with various andsomewhat analogous degrees of difficulty in being grafted
together in order to prevent them becoming inarchedin our forests.
The sterility of first crosses between pure species,
which have their reproductive systems perfect, seems

CHAP. VIII.

SUMMARY.

277

to depend on several circumstances; in some cases
largely on the early death of the embryo. The sterilityof hybrids, which have their reproductive systems imperfect, and which have had this system and their wholeorganisation disturbed by being compounded of two distinct species, seems closely allied to that sterility which
so frequently affects pure species, when their natural
conditions of life have been disturbed. This view is
supported by a parallelism of another kind ;-namely,that the crossing of forms only slightly different isfavourable to the vigour and fertility of their offspring ;and that slight changes in the conditions of life are a pparently favourable to the vigour and fertility of allorganic beings. It is not surprising that the degree ofdifficulty in uniting two species, and the degree ofsterility of their hybrid-offspring should generally correspond, though due to distinct causes ; for both dependon the amount of difference of some kind between thespecies which are crossed. Nor is it surprising thatthe facility of effecting a first cross, the fertility of t,hehybrids produced, and the capacity of being grafted together-though this latter capacity evidently depends
on widely different circumstances-should all run, to acertain extent, parallel with the systematic affinity of
the forms which are subjected to experiment ; for systematic affinity attempts to express all kinds of resemblance between all species.
First crosses between forms known to be varieties, or
sufficiently alike to be considered as varieties, and theirmongrel offspring, are very generally, but not quite universally, fertile. Nor is this nearly general and perfectfertility surprising, when we remember how liable we are
to argue in a circle with respect to varieties in a stateof nature; and when we remember that the greaternumber of varieties have been produced under domesti-

278

HYBRIDISM.

CHAP.VIII.

cation by the selection of mere external differences, andnot of differences in the reproductive system. In allother respects, excluding fertility, there is a close general resemblance between hybrids and mongrels. Finally,
then, the facts briefly given in this chapter do not seemto me opposed to, but even rather to support the view,that there is no fundamental distinction between species
and varieties.

CHAP. JX.

IMPERFECTION OF GEOLOGICAL RECORD.

279

CHAPTER IX.
ON THE ~MPERFECTIONOF THE GEOLOGICALRECORD.
On the absence of intermediate varieties at the present day -On
the nature of extinct intermediate varieties ; on their number
On the vast lapse of time, as inferred from the rate of deposition and of denudation On the poorness of our palaonto!ogicalcollections - On the intermittence of geological formations
On the absence of intermediate varieties in any one formakion
On the sudden appearance of groups of species - On theirsudden appearance in the lowest known fossiliferous strata.

-

-

-

-

IN the sixth chapter I enumerated the chief objections
which might be justly urged against the views maintained in this volume. Most of them have now been
discussed. One, namely the distinctness of specificforms, and their not being blended together by innumerable transitional links, is a very obvious diEiculty.
I assigned reasons why such links do not commonlyoccur at the present day, under the circumstances apparently most favourable for their presence, namely
on an extensive and continuous area with graduatedphysical conditions. I endeavoured to show, that thelife of each species depends in a more important manneron the presence of other already defined organic forms,
than on climate; and, therefore, that the really governing conditions of life do not graduate away quite
insensibly like heat or moisture. I endeavoured, also,
to show that intermediate varieties, from existing inlesser numbers than the forms which they connect, will
generally be beaten out and exterminated during thecourse of further modification and improvement. The
main cause, however, of innumerable intermediate links
not now occurring everywhere throughout nature de-

280

IMPERFECTION OF THE

CHAP. IS.

pends on the very process of natural selection, throughwhich new varieties continually take the places of andexterminate their parent-forms. But just in proportion
as this process of extermination has acted on anenormous scale, so must the number of intermediate
varieties, which have formerly existed on the earth,
be truly enormous. Why then is not every geological formation and every stratum full of such intermediate links ? Geology assuredly does not reveal any
such finely graduated organic chain ; and this, perhaps,is the most obvious and gravest objection which can beurged against my theory. The explanation lies, as I
believe, in the extreme imperfection of the geologicalrecord.
I n the first place it should always be borne in mind
what sort of intermediate forms must, on my theory,
have formerly existed. I have found it difficult,, whenlooking at any two species, to avoid picturing to myself,
forms directly intermediate betweeu them. But thisis a wholly false view ; we should always look for forms
intermediate between each species and a common but
unknown progenitor ; and the progenitor will generally
have differed in some respects from all its modified descendants. To give a simple illustration : the fantail andpouter pigeons have both descended from the rock-pigeon;
if we possessed all the intermediate varieties whichhave ever existed, we should have an extremely close series
between both and the rock-pigeon ; but we should haveno varieties directly intermediate between the fantail
and pouter ; none, for instance, combining a tail somewhat expanded with a crop somewhat enlarged, thecharacteristic features of these two breeds. These twobreeds, moreover, have become so much modified, that
if we had no historical or indirect evidence regarding
their origin, it would not have been possible to have

CEAP. IX.

QEOLOGICAL RECORD.

251

determined from a mere comparison of their structurewith that of the rock-pigeon, whether they had descended
from this species or from some other allied species, such
as C. oenas.
So with natural species, if we look to forms very
distinct, for instance to the horse and tapir, we haveno reason to suppose that links ever existed directlyintermediate between them, but between each and an
unknown common parent. The common parent w i l lhave had in its whole organisation much general resemblance to the tapir and to the horse ; but in some pointsof structure may have differed considerably from both,even perhaps more than they differ from each other.
Hence in all such cases, we should be unable to recognise the parent-form of any two or more species, evenif we closely compared the structure of the parent withthat of its modified descendants, unless at the sametime we had a nearly perfect chain of the intermediate
links.
It is just possible by my theory, that one of two living
forms might have descended from the other; for instance, a horse from a tapir; and in this case directintermediate links will have existed between them.
But such a case would imply that one form had remained for a very long period unaltered, whilst its
descendants had undergone a vast amount of change ;and the principle of competition between organism andorganism, between child and parent, will render this avery rare event ; for in all cases the new and improved
forms of life will tend to supplant the old and unimproved forms.
By the theory of natural selection all living species
have been connected with the parent-species of eachgenus, by differences not greater than we see between the varieties of the same species at the present

282

IMPERFECTION OF THE

CHAP. IX.

day ; and these parent-species, now generally extinct,
have in their turn been similarly connected with moreancient species; and so on backwards, always converging to the common ancestor of each great class.
So that the number of intermediate and transitional
links, between all living and extinct species, must havebeen inconceivably great. But assuredly, if this theory
be true, such have lived upon this earth.

On the lapse of Time.-Independently of our not
finding fossil remains ,of such infinitely numerous connecting links, it may be objected, that time will nothave sufficed for so great an amount of organic change,
all changes having been effected very slowly throughnatural selection. It is hardly possible for me even torecall to the reader, who may not be a practical geologist, the facts leading the mind feebly to comprehend
the lapse of time. He who can read Sir Charles Lyell's
grand work on the Principles of Geology, which the
future historian will recognise aa having produced arevolution in natural science, yet does not admit how
incomprehensibly vast have been the past periods oftime, may at once close this volume. Not that it suffices
to study the Principles of Geology, or to read special
treatises by different observers on separate formations,and to mark how each author attempts to give an inadequate idea of the duration of each formation or eveneach stratum. A man must for years examine forhimself great piles of superimposed strata, and watch
the sea at work grinding down old rocks and making
fresh sediment, before he can hope to comprehend anything of the lapse of time, the monuments of which we
see around us.
It is good to wander along lines of sea-coast,
when formed of moderately hard rocks, and mark theCHAP. 1x.

GEOLOGICAL RECORD.

283

process of degradation. The tides in most cases reachthe cliffs only for a short time twice a day, and thewaves eat into them only when they are charged
with sand or pebbles; for there is reason to believethat pure water can effect little or nothing in wearing
away rock. At last the base of the cliff is undermined, huge fragments fall down, and these remaining fixed, have to be worn away, atom by atom, until
reduced in size they can be rolled about by the waves,and then are more quickly ground into pebbles, sand,or mud. But how often do we see along the bases ofretreating cliffs rounded boulders, all thickly clothedby marine productions, showing how little they areabraded and how seldom they are rolled about ! Moreover, if we follow for a few miles any line of rocky cliff,which is undergoing degradation, we find that it is only
here and there, along a short length or round a promontory, that the cliffs are at the present time suffering.
The appearance of the surface and the vegetation showthat elsewhere years have elapsed since the waters
washed their base.
He who most closely studies the action of the sea on
our shores, will, I believe, be most deeply impressedwith the slowness with which rocky coasts are wornaway. The observations on this head by Hugh Miller,
and by that excellent observer Mr. Smith of Jordan
Hill, are most impressive. With the mind thus impressed, let any one examine beds of conglomeratemany thousand feet in thickness, which, though probably formed at a quicker rate than many other deposits, yet, from being formed of worn and rounded
pebbles, each of which bears the stamp of time, aregood to show how slowly the mass has been accumulated. Let him remember Lyell's profound remark,that the thickness and extent of sedimentary formations

284

IMPERFECTION OF THE

CHAP. IX.

are the result and measure of the degradation which
the earth's crust has elsewhere suffered. And what an
amount of degradation is implied by the sedimentarydeposits of many countries! Professor Ramsay has
given me the maximum thickness, in most cases from
actual measurement, in a few cases from estimate, of
each formation in different parts of Great Britain ; andthis is the result :Feet.

Palsozoic strata, (not including igneous beds)
Secondary strata
Tertiary strata.. 57,154. . . . . . . . . . . . . . 13,190. . . . . . . . . . . . . . 2,240-making altogether 72,584 feet ; that is, very nearly
thirteen and three-quarters British miles. Some ofthese formations, which are represented in Englandby. thin beds, are thousands of feet in thickness on theContinent. Moreover,between each successiveformation,
we have, in the opinion of most geologists, enormously
long blank periods. So that the lofty pile of sedimentary rocks in Britain, gives but an inadequate idea ofthe time which has elapsed during their accumulation ;yet what time this must have consumed! Good observers have estimated that sediment is deposited by thegreat Mississippi river at the rate of only 600 feet in ahundred thousand years. This estimate may be quite
erroneous ; yet, considering over what wide spaces veryfine sediment is transported by the currents of thesea, the process of accumulation in any one area must
be extremely slow.
But the amount of denudation which the strata havein many places sdered, independently of the rate ofaccumulation of the degraded matter, probably offers
the best evidence of the lapse of time. I remember
having been much struck with the evidence of denudat>ion,when viewing volcanic islands, which have been

CHAP.

Ix.

GEOLOGICAL RECORD.

285

worn by the waves and pared all round into perpendicularcliffs of one or two thousand feet in height; for the
gentle slope of the lava-streams, due to their formerly
liquid state, showed at a glance how far the hard, rocky
beds had once extended into the open ocean. The
same story is still more plainly told by faults,-thosegreat cracks along which the strata have been upheaved
on one side, or thrown down on the other, to theheight or depth of thousands of feet; for since the crust
cracked, the surface of the land has been so completely
planed down by the action of the sea, that no trace of
these vast dislocations is externally visible.
The Craven fault, for instance, extends for upwardsof 30 miles, and along this line the vertical displacement of the strata has varied from 600 to 3000 feet.
Prof. Ramsay has published an account of a downthrow
in Anglesea of 2300 feet; and he informs me that hefully believes there is one in Merionethshire of 12,000
feet; yet in these cases there is nothing on the surface
to show such prodigious movements ; the pile of rocks
on the one or other side having been smoothly swept
away. The consideration of these facts impresses mymind almost in the same manner as does the vain endeavour to grapple with the idea of eternity.
I am tempted to give one other case, the well-known
one of the denudation of the Weald. Though it must
be admitted that the denudation of the Weald has beena mere trifle, in comparison with that which has
removed masses of our palsozoic strata, in parts ten
thousand feet in thickness, as shown in Prof. Ramsay’s
masterly memoir on this subject. Yet it is an admirable
lesson to stand on the North Downs and to look at the
distant South Downs; for, remembering that at no
great distance to the west the northern and southern
escarpments meet and close, one can safely picture to286

IN PERFECTION OF THE

CHAP. 1X.

oneself the great dome of rocks which must have covered
up the Weald within so limited a period as since thelatter part of the Chalk formation. The distance fromthe northern to the southern Downs is about 22 miles,
and the thickness of the several formations is on an average about 1100 feet, as I am informed by Prof. Ramsay.
Rut if, as some geologists suppose, a range of older rocks
underlies the Weald, on the flanks of which the overlying sedimentary deposits might have accumulated inthinner masses than elsewhere, the above estimate would
be erroneous ; but this source of doubt probably would
not greatly affect the estimate as applied to the westernextremity of the district. If, then, we knew the rate at
which the sea commonly wears away a line of cliff ofany given height, we could measure the time requisite
to have denuded the Weald. This, of course, cannot
be done; but we may, in order to form Some crude
notion on the subject, assume that the sea would eat
into cliffs 500 feet in height at the rate of one inch
in a century. This will at first appear much too
small an allowance ; but it is the same as if we were to
assume a cliff one yard in height to be eaten backalong a whole line of coast at the rate of one yard innearly every twenty-two years. I doubt whether any
rock, even as soft as chalk, would yield at this rate excepting on the most exposed coasts ; though no doubt thedegradation of a lofty cliff would be more rapid from thebreakage of the fallen fragments. On the other hand,
I do not believe that any line of coast, ten or twenty
miles in length, ever suffers degradation at the same timealong its whole indented length ; and we must rememberthat almost all strata contain harder layers or nodules,
which from long resisting attrition form a breakwaterat the base. Hence, under ordinary circumstances, I
conclude that for a cliff 500 feet in height, a denudation

CHAP.IX.

GEOLOGICAL RECORD.

287

of one inch per century for the whole length would be
an ample allowance. At this rate, on the above
data, the denudation of the Weald must have required
306,662,400 years ; or say three hundred million years.
The action of fresh water on the gently inclined
Wealden district, when upraised, could hardly havebeen great, but it would somewhat reduce the above
estimate. On the other hand, during oscillations oflevel, which we know this area has undergone, the surface may have existed for millions of years as land, andthus have escaped the action of the sea: when deeply
submerged for perhaps equally long periods, it would,
likewise, have escaped the action of the coast-waves.
So that in all probability a far longer period than 300
million years has elapsed since the latter part of theSecondary period.
I have made these few remarks because it is highly
important for us to gain some notion, however imperfect, of the lapse of years. During each of these years,
over the whole world, the land and the water has been
peopled by hosts of living forms. What an infinite
number of generations, which the mind cannot grasp,
must have succeeded each other in the long roll ofyears! Now turn to our richest geological museums,
and what a paltry display we behold!

O n the poornees of our Palceontological co11ectiune.That our palaeontological collections are very imperfect,is admitted by every one. The remark of that admirable palaeontologist, the late Edward Forbes, should not
be forgotten, namely, that numbers of our fossil species
are known and named from single and often broken
specimens, or from a few specimens collected on someone spot. Only a small portion of the surface of theearth has been geologically explored, and no part with

288

IMPERFECTION OF THE

CHAP. IX.

sufficient care, as the important discoveries made every
year in Europe prove. No organism wholly soft can bepreserved. Shells and bones will decay and disappear
when left on the bottom of the sea, where sediment is
not accumulating. I believe we are continually taking
a most erroneous view, when we tacitly admit to ourselves that sediment is being deposited over nearly thewhole bed of the sea, at a rate sd%ciently quick toembed and preserve fossil remains. Throughout anenormously large proportion of the ocean, the brightblue tint of the water bespeaks its purity. The manycases on record of a formation conformably covered,after an enormous interval of time, by another andlater formation, without the underlying bed having
suffered in the interval any wear and tear, seem explicable only on the view of the bottom of the sea notrarely lying for ages in an unaltered condition. The
remains which do become embedded, if in sand or gravel,
will when the beds are upraised generally be dissolvedby the percolation of rain-water. I suspect that but few
of the very many animals which live on the beach between high and low watermark are preserved. For instance, the several species of the Chthamaline (a subfamily of sessile cirripedes) coat the rocks all over theworld in infinite numbers : they are all strictly littoral,
with the exception of a single Mediterranean species,
which inhabits deep water and has been found fossil in
Sicily, whereas not one other species has hitherto beenfound in any tertiary formation: yet it is now known
that the genus Chthamalus existed during the chalk
period. The molluscan genus Chiton offers a partiallyanalogous case.
With respect to the terrestrial productions which
lived during the Secondary and Palaeozoic periods, it issuperfluous to state that our evidence from fossil

CHAP.IX.

CfEOLOCfICAL RECORD.

289

remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging toeither of these vast periods, with one exception discovered by Sir C. Lyell in the carboniferous strata ofNorth America. I n regard to mammiferous remains, asingle glance at the historical table published in the
Supplement to Lyell’s Manual, will bring home thetruth, how accidental and rare is their preservation, farbetter than pages of detail. Nor is their rarity surprising, when we remember how large a proportion of
the bones of tertiary mammals have been discovered
either in caves or in lacustrine deposits ; and that not acave or true lacustrine bed is known belonging to theage of our secondary or palaeozoic formations.
But the imperfection in the geological record mainly
results from another and more important cause than anyof the foregoing ; namely, from the several formations
being separated from each other by wide intervals oftime. When we see the formations tabulated in written
works, or when we follow them in nature, it isdiflicult to avoid believing that they are closely consecutive. But we know, for instance, from Sir R.
Murchison’s great work on Russia, what wide gaps
there are in that country between the superimposed
formations; so it is in North America, and in manyother parts of the world. The most skilful geologist, if
his attention had been exclusively confined to theselarge territories, would never have suspected that during
the periods which were blank and barren in his own
country, great piles of sediment, charged with new andpeculiar forms of life, had elsewhere been accumulated. And if in each separate territory, hardly any
idea can be formed of the length of time which has
elapsed between the consecutive formations, we may inferthat this could nowhere be ascertained. The frequent290

IMPERFECTION OF THE

CHAP. IX.

and great changes in the mineralogical composition ofconsecutive formations, generally implying great changes
in the geography of the surrounding lands, whence thesediment has been derived, accords with the belief ofvast intervals of time having elapsed between each formation.
But we can, I think, see why the geological formations of each region are almost invariably intermittent; that is, have not followed each other in closesequence. Scarcely any fact struck me more when
examining many hundred miles of the South American
coasts, which have been upraised several hundred feetwithin the recent period, than the absence of any recent
deposits sufficiently extensive to last for even a shortgeological period. Along the whole west coast, whichis inhabited by a peculiar marine fauna, tertiary beds
are 80 scantily developed, that no record of several successive and peculiar marine faunas will probably bepreserved to a distant age. A little reflection will explain why along the rising coast of the western side ofSouth America, no extensive formations with recent or
tertiary remains can anywhere be found, though the
supply of sediment must for ages have been great, fromthe enormous degradation of the coast-rocks and frommuddy streams entering the sea. The explanation, no
doubt, is, that the littoral and sub-littoral deposits arecontinually worn away, as soon as they are brought upby the slow and gradual rising of the land within thegrinding action of the coast-waves.
We may, I think, safely conclude that sediment musk
be accumulated in extremely thick, solid, or extensive
masses, in order to withstand the incessant action of
the waves, when first upraised and during subsequent
oscillations of level. Such thick and extenaive accumulations of sediment may be formed in two ways ;either,CHAP.1X.

GEOLOGICAL RECORD.

291

in profound depths of the sea, in which case, judging
from the researches of E. Forbes, we may conclude thatthe bottom will be inhabited by extremely few animals,
and the mass when upraised will give a most imperfectrecord of the forms of life which then existed ; or, sediment may be accumulated to any thickness and extent
over a shallow bottom, if it continue slowly to subside.
I n this latter case, as long as the rate of subsidence
and supply of sediment nearly balance each other,
the sea will remain shallow and favourable for life, andthus a fossiliferous formation thick enough, when u p
raised, to resist any amount of degradation, may beformed.
I am convinced that all our ancient formations,which are rich in fossils, have thus been formed
during subsidence. Since publishing my views on
this subject in 1845, I have watched the progress ofGeology, and have been surprised to note how author
after author, in treating of this or that great formation,
has come to the conclusion that it was accumulated
during subsidence. I may add, that the only ancienttertiary formation on the west coast of South America,
which has been bulky enough to resist such degradation
as it has as yet suffered, but which will hardly last to a
distant geological age, was certainly deposited during adownward oscillation of level, and thus gained considerable thickness.
All geological facts tell us plainly that each area
has undergone numerous slow oscillations of level, andapparently these oscillations have affected wide spaces.
Consequently formations rich in fossils and sufficiently
thick and extensive to resist subsequent degradation,
may have been formed over wide spaces during periodsof subsidence, but only where the supply of sedimentwaa sufficient to keep the sea shallow and to embed and292

IMPERFECTION OF THE

CHAP. IX.

preserve the remains before they had time to decay.
On the other hand, as long as the bed of the sea remained stationary, thick deposits could not have been
accumulated in the shallow parts, which are the mostfavourable to life. Still less could this have happened
during the alternate periods of elevation; or, to speak
more accurately, the beds which were then accumulated will have been destroyed by being upraised andbrought within the limits of the coast-action.
Thus the geological record w i l l almost necessarily berendered intermittent. I feel much confidence in thetruth of these views, for they are in strict accordance withthe general principles inculcated by Sir C. LyeU ; andE. Forbes independently arrived at a similar conclusion.
One remark is here worth a passing notice. During
periods of elevation the area of the land and of theadjoining shoal parts of the sea will be increased, andnew stations will often be formed ;-all circumstancesmost favourable, as previously explained, for the formation of new varieties and species; but during such
periods there w i l l generally be a blank in the geologicalrecord. On the other hand, during subsidence, theinhabited area and number of inhabitants will decrease(excepting the productions on the shores of a continent
when &st broken up into an archipelago), and consequently during subsidence, though there will be muchextinction, fewer new varieties or species will be formed ;and it is during these very periods of subsidence, thatour great deposits rich in fossils have been accumulated.
Nature may almost be said to have guarded against thefrequent discovery of her transitional or linking forms.
From the foregoing considerations it cannot be doubted
that the geological record, viewed as a whole, is extremely imperfect; but if we confine our attention to
any one formation, it becomes more difficult to under-CHAP.

GEOLOGICAL RECORD.

293

stand, why we do not therein find closely graduatedvarieties between the allied species which lived at its
commencement and at its close. Some cases are on
record of the same species presenting distinct varieties
in the upper and lower parts of the same formation, but,as they are rare, they may be here passed over. Although each formation has indisputably required avast number of years for ib deposition, I can see several
reasons why each should not include a graduated seriesof links between the species which then lived ; but I can
by no means pretend to assign due proportional weightto the following considerations.
Although each formation may mark a very long lapseof years, each perhaps is short compared with the periodrequisite to change one species into another. I amaware that two pakeontologists, whose opinions areworthy of much deference, namely Bronn and Woodward, have concluded that the average duration of eachformation is twice or thrice as long as the average
duration of specific forms, But insuperable difficulties,
as it seems to me, prevent us coming to any just conclusion on this head. When we see a species first appearing in the middle of any formation, it would be rash
in the extreme to infer that it had not elsewhere previously existed. So again when we find a species disappearing before the uppermost layers have been deposited,
it would be equally rash to suppose that it then became
wholly extinct. We forget how small the area of Europe is compared with the rest of the world ; nor havethe several stages of the same formation throughout
Europe been correlated with perfect accuracy.
With marine animals of all kinds, we may safely
infer a large amount of migration during climataland other changes; and when we see a species first
appearing in any formation, the probability is that it294

IMPERFECTION OF THE

CHAP.IX.

only then first immigrated into that area. It is wellknown, for instance, that several species appeared somewhat earlier in the palreozoic beds of North America
than in those of Europe ; time having apparently beenrequired for their migration from the American to theEuropean seas. In examining the latest deposits ofvarious quarters of the world, it has everywhere been
noted, that some few still existing species are common
in the deposit, but have become extinct in the immediately surrounding sea; or, conversely, that some are
now abundant in the neighbouring sea, but are rare orabsent in this particular deposit. It is an excellent
lesson to reflect on the ascertained amount of migrationof the inhabitants of Europe during the Glacial period,
which forms only a part of one whole geological period ;and likewise to reflect on the great changes of level,on the inordinately great change of climate, on theprodigious lapse of time, all included within this same
glacial period. Yet it may be doubted whether in any
quarter of the world, sedimentary deposits, including
fossil remains, have gone on accumulating within thesame area during the whole of this period. It is not,for instance, probable that sediment was deposited during the whole of the glacial period near the mouth of
the Mississippi, within that limit of depth at which marine animals can flourish; for we know what vast geographical changes occurred in other parts of America
during this space of time. When such beds as were
deposited in shallow water near the mouth of the Mississippi during some part of the glacial period shall havebeen upraised, organic remains will probably first appear
and disappear at different levels, owing to the migrationof species and to geographical changes. And in the
distant future, a geologist examining these beds, might
be tempted to conclude that the average duration of life

CHAP.IX.

GEOLOGICAL RECORD.

295

of the embedded fossils had been less than that of theglacial period, instead of having been really far greater,that is extending from before the glacial epoch to thepresent day.
I n order to get a perfect gradation between two forms
in the upper and lower parts of the same formation, thedeposit must have gone on accumulating for a very long
period, in order to have given s a c i e n t time for theslow process of variation ; hence the deposit will generally have to be a very thick one ; and the species undergoing modification will have had to live on the same
area throughout this whole time. But we have seenthat a thick fossiliferous formation can only be accumulated during a period of subsidence ; and to keep the
depth approximately the same, which is necessary inorder to enable the same species to live on the same
space, the supply of sediment must nearly have counterbalanced the amount of subsidence. But this same
movement of subsidence w i l l often tend to sink thearea whence the sediment is derived, and thus diminish
the supply whilst the downward movement continues.
I n fact, this nearly exact balancing between the supplyof sediment and the amount of subsidence is probably
a rare contingency; for it has been observed by morethan one palaeontologist, that very thick deposits areusually barren of organic remains, except near their
upper or lower limits.
It would seem that each separate formation, like thewhole pile of formations in any country, h s generally
been intermittent in its accumulation. When we see,as is so often the case, a formation composed of bedsof different mineralogical composition, we may reasonably suspect that the process of deposition has beenmuch interrupted, as a change in the currents of thesea and a supply of sediment of a different nature w i l l296

IMPERFECTION OF THE

CHAP.I L

generally have been due to geographical changes requiring much time. Nor w i l l the closest inspection of
a formation give any idea of the time which its deposition has consumed. Many instances could be given ofbeds only a few feet in thickness, representing formations, ekewhere thousands of feet in thickness, andwhich must have required an enormous period for theiraccumulation; yet no one ignorant of this fact would
have suspected the vast lapse of time represented by
the thinner formation. Many cases could be given of
the lower beds of a formation having been upraised,denuded, submerged, and then re-covered by the upper
beds of the same formation,-facts, showing what wide,
yet easily overlooked, intervals have occurred in its accumulation. I n other cases we have the plainest evidence
in great fossilised trees, still standing upright as theygrew, of many long intervals of time and changes oflevel during the process of deposition, which would
never even have been suspected, had not the trees
chanced to have been preserved: thus, Messrs. Lyell
and Dawson found carboniferous beds 1400 feet thickin Nova Scotia, with ancient root-bearing Arata, one
above the other, at no less than sixty-eight different
levels. Hence, when the same species occur at thebottom, middle, and top of a formation, the probability
is that they have not lived on the same spot during thewhole period of deposition, but have disappeared andreappeared, perhaps many times, during the same geological period. So that if such species were to undergo
a considerable amount of modification during any one
geological period, a section would not probably include
all the fine intermediate gradations which must on mytheory have existed between them, but abrupt, thoughperhaps very slight, changes of form.
It is all-important to remember that naturalists have

CHAP.IX.

GEOLOGICAL RECORD.

297

no golden rule by which to distinguish species and varieties ; they grant some little variability to each species,
but when they meet with a somewhat greater amountof difference between any two forms, they rank. both as
species, unless they are enabled to connect them together by close intermediate gradations. And this fromthe reasons just assigned we can seldom hope to effectin any one geological section. Supposing B and C to
be two species, and a third, A, to be found in anunderlying bed; even if A were strictly intermediate
between B and C, it would simply be ranked as a thirdand distinct species, unless at-the same time it could be
most closely connected with either one or both forms byintermediate varieties. Nor should it be forgotten, as
before explained, that A might be the actual progenitorof B and C, and yet might not at all necessarily bestrictly intermediate between them in all points of structure. So that we might obtain the parent-species andits several modified descendants from the lower andupper beds of a formation, and unless we obtained
numerous transitional gradations, we should not recognise their relationship, and should consequently be compelled to rank them all as distinct species.
It is notorious on what excessively slight differences
many paleontologists have founded their species ; andthey do this the more readily if the specimens comefrom Merent sub-stages of the same formation. Someexperienced conchologists are now sinking many of thevery fine species of D’Orbigny and others into the rankof varieties; and on this view we do find the kind ofevidence of change which on my theory we ought tofind. Moreover, if we look to rather wider intervals,
namely, to distinct but consecutive stages of the same
great formation, we find that the embedded fossils,though almost universally ranked as specificallydifferent,

298

IMPERFECTION OF THE

CHAP.IX.

yet are far more closely allied to each other thanare the species found in more widely separated formations ; but to this subject I shall have to return in thefollowing chapter.
One other consideration is worth notice : with animals
and plants that can propagate rapidly and are nothighly locomotive, there is reason to suspect, as wehave formerly seen, that their varieties are generally at
first local ; and that such local varieties do not spreadwidely and supplant their parent-forms until they havebeen modified and perfected in some considerable degree. According to this view, the chance of discoveringin a formation in any one country all the early stagesof transition between any two forms, is small, for the
successive changes are supposed to have been local orcodned to some one spot. Most marine animals havea wide range ; and we have seen that with plants it isthose which have the widest range, that oftenest present
varieties ; so that with shells and other marine animals,
it is probably those which have had the widest range,
far exceeding the limits of the known geological formations of Europe, which have oftenest given rise, firstto local varieties and ultimately to new species; andthis again would greatly lessen the chance of our being
able to trace the stages of transition in any one geological formation.
It should not be forgotten, that at the present day,with perfect specimens for examination, two forms canseldom be connected by intermediate varieties and thus
proved to be the same species, until many specimens
have been collected from many places ; and in the caseof fossil species this could rarely be effected by paleontologists. We shall, perhaps, best perceive the improbability of our being enabled to connect species by
numerous, fine, intermediate, fossil links, by asking

CHAP. IX.

GEOLOGICAL RECORD.

299

ourselves whether, for instance, geologists at somefuture period w i l l be able to prove, that our different
breeds of cattle, sheep, horses, and dogs have descended
from a single stock or from several aboriginal stocks ;or, again, whether certain sea-shells inhabiting theshores of North America, which are ranked by some conchologists as distinct species from their European representatives, and by other conchologists as only varieties,
are really varieties or are, as it is called, specificallydistinct. This could be effected only by the future geologist. discovering in a fossil state numerous intermediate
gradations ; and such success seems to me improbable in
the highest degree.
Geological research, though it has added numerous
species to existing and extinct genera, and has made theintervals between some few groups less wide than they
otherwise would have been, yet has done scarcely anything in breaking down the distinction between species,
by connecting them together by numerous, fine, intermediate varieties; and this not having been effected,
is probably the gravest and most obvious of all the
many objections which may be urged against my views.
Hence it will be worth while to sum up the foregoingremarks, under an imaginary illustration. The Malay
Archipelago is of about the size of Europe from theNorth Cape to the Mediterranean, and from Britain to
Russia ; and therefore equals all the geological formations which have been examined with any accuracy,
excepting those of the United States of America.
I fully agree with Mr. Godwin-Austen, that thepresent condition of the Malay Archipelago, with its
numerous large islands separated by wide and shallow
seas, probably represents the former state of Europe,
when most of our formations were accumulating. The
Malay Archipelago is one of the richest regions of the

300

IMPERFECTION OF THE

CHAP. IX.

whole world in organic beings; yet if all the species
were to be collected which have ever lived there, how
imperfectly would they represent the natural history of
the world I
But we have every reason to believe that the terrestrial productions of the archipelago would be preservedin an excessively imperfect manner in the formations
which we suppose to be there accumulating. I suspectthat not many of the strictly littoral animals, or of
those which lived on naked submarine rocks, would
be embedded ; and those embedded in gravel or sand,would not endure to a distant epoch. Wherever sediment did not accumulate on the bed of the sea, or where
it did not accumulate at a sufEcient rate to protectorganic bodies from decay, no remains could be preserved.
In our archipelago, I believe that fossiliferous formations could be formed of sufficient thickness to last to
an age, as distant in futurity as the secondary formations lie in the past, only during periods of subsidence.
These periods of subsidence would be separated fromeach other by enormous intervals, during which thearea would be either stationary or rising ; whilst rising,each fos'siliferous formation would be destroyed, almost
as soon as accumulated, by the incessant coast-action, aa
we now see on the shores of South America. During
the periods of subsidence there would probably be muchextinction of life ; during the periods of elevation, there
would be much variation, but the geological record
would then be least perfect.
It may be doubted whether the duration of any one
great period of subsidence over the whoIe or part of
the archipelago, together with a contemporaneous mumulation of sediment, would exceed the average durationof the same specific forms ; and these conhhgencies are

CHAP. Ix.

GEOLOQICAL RECORD.

301

indispensable for the preservation of all the transit,ional
gradations between any two or more species. If suchgradations were not fully preserved, transitional varieties
would merely appear as so many distinct species. It is,also, probable that each great period of subsidence would
be interrupted by oscillations of level, and that slight
climatal changes would intervene during such lengthy
periods ; and in these cases the inhabitants of the archipelago would have to migrate, and no closely consecutive record of their modifications could be preserved inany one formation.
Very many of the marine inhabitants of the archipelago now range thousands of miles beyond its confines ;and analogy leads me to believe that it would be chiefly
these far-ranging species which would oftenest produce
new varieties; and the varieties would at first generally be local or confined to one place, but if possessedof any decided advantage, or when further modified andimproved, they would slowly spread and supplant theirparent-forms. When such varieties returned to theirancient homes, as t,hey would differ from their former
state, in a nearly uniform, though perhaps extremely
slight degree, they would, according to the principles
followed by many palzontologists, be ranked as new anddistinct species.
If then, there be some degree of truth in .these
remarks, we have no right to expect to find in our
geological formations, an infinite number of those finetransitional forms, which on my theory assuredly haveconnected all the past and present species of the samegroup into one long and branching chain of life. We
ought only to look for a few links, some more closely,some more distantly related to each other; and theselinks, let them be ever so close, if found in different
stages of the same formation, would, by most palEont0-

302

IMPERFECTION' OF THE

CHAP. IX.

logists, be ranked as distinct species. But I do not pretend that I should ever have suspected how poor arecord of the mutations of life, the best preserved
geological section presented, had not the difficulty of
our not discovering innumerable transitional links
between the species which appeared at the commencement and close of each formation, pressed so hardly
on my theory.

On the sudden appearance of whobe groups of Allied
Species.-The
abrupt manner in which whole groups ofspecies suddenly appear in certain formations, has been
urged by several palaeontologists, for instance, byAgassiz, Pictet, and by none more forcibly than byProfessor Sedgwick, as a fatal objection to the belief inthe transmutation of species. If numerous species,
belonging to the same genera or families, have really
started into life all at once, the fact would be fatal to
the theory of descent with slow mod5cation throughnatural selection. For the development of a groupof forms, all of which have descended from someone progenitor, must have been an extremely slow
process ; and the progenitors must have lived long ages
before their modified descendants. But we continually
over-rate the perfection of the geological record, andfalsely infer, because certain genera or families havenot been found beneath a certain stage, that they did
not exist before that stage. We continually forget
how large the world is, compared with the area overwhich our geological formations have been carefully
examined ; we forget that groups of species may elsewhere have long existed and have slowly multiplied
before they invaded the ancient archipelagoes of Europe
and of the United States. We do not make due allowance for the enormous intervals of time, which have

CHAP.1X.

GEOLOGICAL RECORD.

303

probably elapsed between our consecutive formations,longer perhaps in some cases than the time required
for the accumulation of each formation. These intervals
will have given time for the multiplication of species
from some one or some few parent-forms; and in the
succeeding formation such species will appear as if suddenly created.
I may here recall a remark formerly made, namelythat it might require a long succession of ages to adapt
an organism to some new and peculiar line of life, forinstance to fly through the air ; but that when this had
been effected, and a few species had thus acquired agreat advantage over other organisms, a comparatively
short time would be necessary to produce many divergent forms, which would be able to spread rapidly
and widely throughout the world.
I will now give a few examples to illustrate these
remarks; and to show how liable we are to error insupposing that whole groups of species have suddenly
been produced. I may recall the well-known fact thatin geological treatises, published not many years ago,the great class of mammals was always spoken of aa
having abruptly come in at the commencement of thetertiary series. And now one of the richest known
accumulations of fossil mammals belongs to the middleof the secondary series; and one true mammal has
been discovered in the new red sandstone at nearly thecommencement of this great series. Cuvier used tourge that no monkey occurred in any tertiary stratum ;but now extinct species have been discovered in India,
South America, and in Europe even as far back as the
eocene stage. The most striking case, however, is thatof the Whale family; as these animals have huge bones,
are marine, and range over the world, the fact of nota single bone of a whale having been discovered in

304

IMPERFECTION OF THE

CHAP. 1X.

any secondary formation, seemed fully to justify the
belief that this great and distinct order had been suddenly produced in the interval between the latest
secondary and earliest tertiary formation. But now we
may read in the Supplement to Lyell's ' Manual,' published in 1858, clear evidence of the existence of whalesin the upper greensand, some time before the close of
the secondary period.
I may give another instance, which from having
passed under my own eyes has much struck me. In amemoir on Fossil Sessile Cirripedes, I have stated that,from the number of existing and extinct tertiary species ;from the extraordinary abundance of the individuals of
many species all over the world, from the Arctic regions
to the equator, inhabiting various zones of depths fromthe upper tidal limits to 50 fathoms ; from the perfectmanner in which specimens are preserved in the oldest
tertiary beds; from the ease with which even a fragment of a valve can be recognised; from all these circumstances, I inferred that had sessile cirripedes existed
during the secondary periods, they would certainly havebeen preserved and discovered ; and as not one species
had been discovered in beds of this age, I concluded
that this great group had been suddenly developed at
the commencement of the tertiary series. This was asore trouble to me, adding as I thought one more instance of the abrupt appearance of a great group of
species. But my work had hardly been published, whena skilful palaeontologist, M. Bosquet, sent me a drawing
of a perfect specimen of an unmistakeable sessile cirripede, which he had himseIf extracted from the chalkof Belgium. And, as if to make the case as striking as
possible, this sessile cirripede was a Chthamalus, a very
common, large, and ubiquitous genus, of which not one
specimen has a5 yet been found even in any tertiaryCEAP. Ix.

GEOLOGICAL RECORD.

305

stratum. Hence we now positively know that sessile
cirripedes existed during the secondary period ; andthese cirripedes might have been the progenitors of ourmany tertiary and existing species.
The case most frequently insisted on by palaeontologists of the apparently sudden appearance of a wholegroup of species, is that of the teleostean fishes, lowdown in the Chalk period. This group includes thelarge majority of existing species. Lately, Professor
Pictet has carried their existence one sub-stage further
back ; and some palaeontologists believe that certain
much older fishes, of which the afhities are as yet imperfectly known, are really teleostean. Assuming, however, that the whole of them did appear, as Agassiz
believes, at the commencement of the chalk formation,
the fact would certainly be highly remarkable; but I
cannot see that it would be an insuperable difficulty on
my theory, unless it could likewise be shown that the
species of this group appeared suddenly and simultaneously throughout the world at this same period. It
is almost superfluous to remark that hardly any fossilfish are known from south of the equator; and byrunning through Pictet’s Palaeontology it will be seenthat very few species are known from several formations
in Europe. Some few families of fish now have a
confined range; the teleostean fish might formerly
have had a similarly confined range, and after having
been largely developed in some one sea, might havespread widely. Nor have we any right to suppose thatthe seas of the world have always been so freely openfrom south to north 88 they are at present. Even at
this day, if the Malay Archipelago were converted into
land, the tropical parts of the Indian Ocean would forma large and perfectly enclosed basin, in which any greatgroup of marine animals might be multiplied; and306

IMPERFECTIOY OF THE

CHAP. JX.

here they would remain confined, until some of thespecies became adapted to a cooler climate, and wereenabled to double the southern capes of Africa orAustralia, and thus reach other and distant seas.
From these and similar considerations, but chiefly
from our ignorance of the geology of other countries
beyond the confines of Europe and the United States ;and from the revolution in our palaeontological ideas
on many points, which the discoveries of even the last
dozen years have effected, it seems to me to be about
as rash in us to dogmatize on the succession of organic
beings throughout the world, as it would be for anaturalist to land for five minutes on some one barren
point in Australia, and then to discuss the number
and range of its productions,
On the sudden appearance of groups of Allied Species in
the lowest known f ossiliferous strata.-There is anotherand allied difficulty, which is much graver. I allude
to the manner in which numbers of species of the samegroup, suddenly appear in the lowest known fossiliferous
rocks. Most of the arguments which have convincedme that all the existing species of the same group havedescended from one progenitor, apply with nearly equal
force to the earliest known species. For instance, I
cannot doubt that all the Silurian trilobites havedescended from some one crustacean, which must have
lived long before the Silurian age, and which probably
differed greatly from any known animal. Some of
the most ancient Silurian animals, as the Nautilus,
Lingula, &c., do not differ much from living species;
and it cannot on my theory be supposed, that these old
species were the progenitors of all the species of theorders to which they belong, for they do not presentcharacters in any degree intermediate between them.

CHAP. IX.

GEOLOGICAL RECORD.

307

If, moreover, they had been the progenitors of theseorders, they would almost certainly have been long ago
supplanted and exterminated by their numerous andimproved descendants.
Consequently, if my theory be true, it is indisputablethat before the lowest Silurian stratum was deposited,long periods elapsed, as long as, or probably far longer
than, the whole interval from the Silurian age to thepresent day ; and that during these vast, yet quite unknown, periods of time, the world swarmed with living
creatures.
To the question why we do not find records of thesevast primordial periods, I can give no satisfactory
answer. Several of the most eminent geologists, with
Sir R. Murchison at their head, are convinced that we
see in the organic remains of the lowest Silurian stratum
the dawn of life on this planet. Other highly competent judges, as Lyell and the late E. Forbes, dispute
this conclusion. We should not forget that only a small
portion of the world is known with accuracy. M. Barrande has lately added another and lower stage to the
Silurian system, abounding with new and peculiar
species. Traces of life have been detected in the Longmynd be& beneath Barrande’s so-called primordialzone. The presence of phosphatic nodules and bituminous matter in some of the lowest azoic rocks, probably indicates the former existence of life at these
periods. But the difficulty of understanding the absence of vast piles of fossiliferous strata, which on mytheory no doubt were somewhere accumulated before
the Silurian epoch, is very great. If these most ancient
beds had been wholly worn away by denudation, orobliterated by metamorphic action, we ought to find
only small remnants of the formations next succeedingthem in age, and these ought to be very generally in308

IMPERFECTION OF THE

c w . IX.

a metamorphosed condition. But the descriptions which
we now possess of the Silurian deposits over immense
territories in Russia and in North America, do not support the view, that the older a formation is, the more it
has suffered the extremity of denudation and metamorphism.
The case at present must remain inexplicable ; andmay be truly urged as a valid argument against theviews here entertained. To show that it may hereafter
receive some explanation, I will give the following
hypothesis. From the nature of the organic remains,which do not appear to have inhabited profounddepths, in the several formations of Europe and of theUnited States ; and from the amount of sediment, milesin thickness, of which the formations are composed, we
may infer that from first to last large islands or tractsof land, whence the sediment was derived, occurred in
the neighbourhood of the existing continents of Europe
and North America. But we do not know what was
the state of things in the intervals between the successive formations ; whether Europe and the UnitedStates during these intervals existed as dry land, or
as a submarine surface near land, on which sediment
was not deposited, or again as the bed of an open andunfathomable sea.
Looking to the existing oceans, which are thrice as
extensive as the land, we see them studded with manyislands; but not one oceanic island is as yet known
to afford even a remnant of any paleozoic or secondary
formation. Hence we may perhaps infer, that during
the paleozoic and secondary periods, neither continents
nor continental islands existed where our oceans now
extend; for had they existed there, paleozoic andsecondary formations would in all probability have been
accumulated from sediment derived from t,heir wear andCHAP. IY.

GEOLOGICAL RECORD.

309

tear ; and would have been at least partially upheavedby the oscillations of level, which we may fairly conclude must have intervened during these enormously
long periods. If then we may infer anything fromthese facts, we may infer that where our oceans now
extend, oceans have extended from the remotest periodof which we have any record ; and on the other hand,that where continents now exist, large tracts of land
have existed, subjected no doubt to great oscillations oflevel, since the earliest Silurian period. The coloured
map appended to my volume on Coral Reefs, led me toconclude that the great oceans are still mainly areas of
subsidence, the great archipelagoes still areas of oscillations of level, and the continents areas of elevation.
But have we any right to assume that things have thus
remained from eternity ? Our continents seem to havebeen formed by a preponderance, during many oscillations of level, of the force of elevation ; but may not theareas of preponderant movement have changed in thelapse of ages? At a period immeasurably antecedent to
the Silurian epoch, continents may have existed where
Oceans are now spread out; and clear and open oceans
may have existed where our continents now stand. Norshould we be justified in assuming that if, for instance,
the bed of the Pacific Ocean were now converted into
a continent, we should there find formations older
than the Silurian strata, supposing such to have been
formerly deposited ; for it might well happen that strata
which had subsided some miles nearer to the centre of
the earth, and which had been pressed on by an enormous weight of superincumbent water, might haveundergone far more metamorphic action than strata
which have always remained nearer to the surface. The
immense areas in some parts of the world, for instancein South America, of bare metamorphic rocks, which

310

IMPERFECTION OF T H E

CHAP. 1x.

must have been heated under great pressure, havealways seemed to me to require some special explanstion ; and we may perhaps believe that we see in theselarge areas, the many formations long anterior to the
Silurian epoch in a completely metamorphosed condition.
The several rnculties here discussed, namely our not
finding in the successive formations infinitely numerous
transitional links between the many species which now
exist or have existed; the sudden manner in whichwhole groups of species appear in our European formations ; the almost entire absence, as at present known,of fossiliferous formations beneath the Silurian strata,
are all undoubtedly of the gravest nature. We Bee thisin the plainest manner by the fact that all the mosteminent paleontologists, namely Cuvier, Owen, Agassiz,
Barrande, Falconer, E. Forbes, &c., and all our greatest
geologists, as Lyell, Murchison, Sedgwick, &c., haveunanimously, often vehemently, maintained the immutability of species. But I have reason to believe that one
great authority, Sir Charles Lyell, from further reflexion entertains grave doubta on this subject. I feel
how rash it is to differ from these great authorities,
to whom, with others, we owe all our knowledge.
Those who think the natural geological record inany degree perfect, and who do not attach muchweight to the facts and arguments of other kinds
given in this volume, will undoubtedly at once reject my theory. For my part, following out Lyell's
metaphor, I. look at the natural geological record, as ahistory of the world imperfectly kept, and written ina changing dialect ; of this history we possess the last
volume alone, relating only to two or three countries.
Of this volume, only here and there a short chapter has

CHAP.IX.

GEOLOGICAL RECORD.

311

been preserved ; and of each page, only here and therea few lines. Each word of the slowly-changing language, in which the history is supposed to be written,
being more or less different in the interrupted succession of chapters, may represent the apparently
abruptly changed forms of life, entombed in our consecutive, but widely separated, formations. On thisview, the difficulties above discussed are greatly diminished, or even disappear.

312

GEOLOGJCAL SUCCESSION.

CHAP.X.

CHAPTER X.
ON THE GEOLOGICALSUCCESSION
OF ORGANICBEINGS.
On the slow and successive appearance of new species - On theirdifferent rates of change - Species once lost do not reappear Groups of species follow the same general rules in their appearance and disappearance as do single species -On Extinction On simultaneous changes in the forms of life throughout theworld - On the affinities of extinct species to each other and toliving species -On the state of development of ancient forms On the succession of the same types within the same areas Summary of preceding and present chapters.

LET us now see whether the several facts and rules
relating to the geological succession of organic beings,
better accord with the common view of the immutabilityof species, or with that of their slow and gradual modification, through descent and natural selection.
New species have appeared very slowly, one after
another, both on the land and in the waters. Lye11
has shown that it is hardly possible to resist the evidence
on this head in the case of the several tertiary stages ;and every year tends to fill up the blanks between them,and to make the percentage system of lost and newforms more gradual. I n some of the most recent beds,
though undoubtedly of high antiquity if measured by
years, only one or two species are lost forms, and only
one or two are new forms, having here appeared forthe first time, either locally, or, as far as we know, on
the face of the earth. If we may trust the observationsof Philippi in Sicily, the successive changes in the marine
inhabitants of that island have been many and most
gradual. The secondary formations are more broken ;but, as Bronn has remarked, neither the appearanceCHAP.X.

GEOLOGICAL SUCCESSION.

313

nor disappearance of their many now extinct species has
been simultaneous in each separate formation.
Species of different genera and classes have notchanged at the same rate, or in the same degree. In
the oldest tertiary beds a few living shells may still befound in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact,
in an existing crocodile associated with many strange
and lost mammals and reptiles in the sub-Himalayan
deposits. The Silurian Lingula differs but little fromthe living species of this genus ; whereas most of theother Silurian Molluscs and all the Crustaceans havechanged greatly. The productions of the land seem tochange at a quicker rate than those of the sea, of whicha striking instance has lately been observed in Switzerland. There is some reason to believe that organisms,
considered high in the scale of nature, change more
quickly than those that are low : though there are exceptions to this rule. The amount of organic change,
as Pictet has remarked, does not strictly correspondwith the succession of our geological formations ; so thatbetween each two consecutive formations, the forms oflife have seldom changed in exactly the same degree.
Yet if we compare any but the most closely related formations, all the species will be found to have undergone
some change. When a species has once disappeared
from the face of the earth, we have reason to believethat the same identical form never reappears. The
strongest apparent exception to this latter rule, is thatof the so-called '' colonies " of M. Barrande, which
intrude for a period in the midst of an older formation.
and then allow the pre-existing fauna to reappear ; but
Lyell's explanation, namely, that it is a case of temporary migration from a distinct geographical province,
seems to me satisfactory.

314

GEOLOGICAL SUCCESSION.

CHAP.

s.

These several facts accord well with my theory. I
believe in no fixed law of development, causing all the
inhabitants of a country to change abruptly, or simultaneously, or to an equal degree. The process of modification must be extremely slow. The variability of eachspecies is quite independent of that of all others.
Whether such variability be taken advantage of by
natural selection, and whether the variations be accumulated to a greater or lesser amount, thus causing
a greater or lesser amount of modification in the varying species, depends on many complex contingencies,-on the variability being of a beneficial nature, on
the power of intercrossing, on the rate of breeding,on the slowly changing physical conditions of thecountry, and more especially on the nature of the otherinhabitants with which the varying species comes into
competition. Hence it is by no means surprising thatone species should retain the same identical form much
longer than others ; or, if changing, that it should change
less. We see the same fact in geographical distribution ;for instance, in the land-shells and coleopterous insectsof Madeira having come to differ considerably from theirnearest allies on the continent of Europe, whereas themarine shells and birds have remained unaltered. We
can perhaps understand the apparently quicker rate ofchange in terrestrial and in more highly organised productions compared with marine and lower productions,by the more complex relations of the higher beingst o their organic and inorganic conditions of life, as
explained in a former chapter. When many of the
inhabitants of a country have become modified and improved, we can understand, on the principle of competition, and on that of the many all-important relations of organism to organism, that any form which
does not become in some degree modified and improved,

CHAP.

x.

GEOLOGICAL SUCCESSION.

315

will be liable to be exterminated. Hence we cansee why all the species in the same region do at last,
if we look to wide enough intervals of time, become
modified ; for those which do not change will become
extinct.
I n members of the same class the average amount ofchange, during long and equal periods of time, may,perhaps, be nearly the same ; but as the accumulationof long-enduring fossiliferous formations depends on
great masses of sediment having been deposited on
areas whilst subsiding, our formations have been almost
necessarily accumulated at wide and irregularly intermittent intervals ; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations is not equal. Each formation, on
this view, does not mark a new and complete' act of
creation, but only an occasional scene, taken almost athazard, in a slowly changing drama.
We can clearly understand why a species when once
lost should never reappear, even if the very same conditions of life, organic and inorganic, should recur.
For though the offspring of one species might beadapted (and no doubt this has occurred in innumerable instances) to fill the exact place of anotherspecies in the economy of nature, and thus supplant it ;yet the two forms-the old and the new-would not
be identically the same; for both would almost certainly inherit different characters from their distinct
progenitors. For instance, it is just possible. if our
fantail-pigeons were all destroyed, that fanciers, bystriving during long ages for the same object, mightmake a new breed hardly distinguishable from ourpresent fantail ; but if the parent rock-pigeon were also
destroyed, and in nature we have every reason to believethat the parent-form will generally be supplanted and316

GEOLOGICAL SUCCESSION.

CHAP.X.

exterminated by its improved offspring, it is quite incredible that a fantail, identical with the existing breed,
could be raised from any other species of pigeon, oreven from the other well-established races of the domestic pigeon, for the newly-formed fantail would bealmost, sure to inherit from its new progenitor someslight characteristic differences.
Groups of species, that is, genera and families, follow
the same general rules in their appearance and disappearance as do single species, changing more or lessquickly, and in a greater or lesser degree. A groupdoes not reappear after it bas once disappeared; or
its existence, as long as it lasts, is continuous. I amaware that there are some apparent exceptions to thisrule, but the exceptions are surprisingly few, so
few, that E. Forbes, Pictet, and Woodward (though allstrongly opposed to such views as I maintain) admit
its truth ; and the rule strictly accords with my theory.
For as all the species of the same group have descended
from some one species, it is clear that as long as any
species of the group have appeared in the long successionof ages, so long must its members have continuously
existed, in order to have generated either new andmodified or the same old and unmodified forms. Speciesof the genus Lingula, for instance, must have continuously existed by an unbroken succession of generations,from the lowest Silurian stratum to the present day.
We have seen in the last chapter that the speciesof a group sometimes falsely appear to have come inabruptly; and I have attempted to give an explanation of this fact, which if true would have been fatal
to my views. But such cases are certainly exceptional; the general rule being a gradual increase in
number, till the group reaches its maximum, andthen, sooner or later, it gradually decreases. If theCHAP.

x.

EXTINCTION.

317

number of the species of a genus, or the number ofthe genera of a family, be represented by a verticalline of varying thickness, crossing the successive geological formations in which the species are found, theline will sometimes falsely appear to begin at its lower
end, not in a sharp point, but abruptly ; it then gradually thickens upwards, sometimes keeping for a spaceof equal thickness, and ultimately thins out in theupper beds, marking the decrease and final extinctionof the species. This gradual increase in number of thespecies of a group is strictly conformable with mytheory; as the species of the same genus, and thegenera of the same family, can increase only slowly andprogressively ; for the process of modification and the
production of a number of allied forms must be slow
and gradual,-one species giving rise first to two or
three varieties, these being slowly converted into species,
which in their turn produce by equally slow steps otherspecies, and so on, like the branching of a great tree
from a single stem, till the group becomes large.
On Extinction.-We have as yet spoken only incidentally of the disappearance of species and of groupsof species. On the theory af natural selection the extinction of old forms and the production of new and improved forms are intimately connected together. The
old notion of all the inhabitants of the earth having
been swept away at successive periods by catastrophes,
is very generally given up, even by those geologists,
as Elie de Beaumont, Murchison, Barrande, &c., whose
general views would naturally lead them to this conclusion. On the contrary, we have every reason to believe,
from the study of the tertiary formations, that species
and groups of species gradually disappear, one after
another, first from one spot, then from another, and318

GEOLOCIICAL SUCCESSION.

CHAP.

x.

finally from the world. Both single species and wholegroups of species last for very unequal periods; somegroups, as we have seen, having endured from the earliest known dawn of life to the present day; some having
disappeared before the close of the palaeozoic period.
No fixed law seems to determine the length of time
during which any single species or any single genus
endures. There is reason to believe that the complete
extinction of the species of a group is generally aslower process than their production : if the appearance
and disappearance of a group of species be represented,
as before, by a vertical line of varying thickness, theline is found to taper more gradually at its upper end,
which marks the progress of extermination, than at its
lower end, which marks the first appearance and increase in numbers of the species. I n some cases, however, the extermination of whole groups of beings, as ofammonites towards the close of the secondary period,has been wonderfully sudden.
The whole subject of the extinction of species has
been involved in the most gratuitous mystery. Someauthors have even supposed that as the individual has adefinite length of life, so have species a definite duration. No one I think can have marvelled more at theextinction of species, than I have done. When I foundin La Plata the tooth of a horse embedded with theremains of Mastodon, Megatherium, Toxodon, and otherextinct monsters, which all co-existed with still living
shells at a very late geological period, I was filled withastonishment ; for seeing that the horse, since its introduction by the Spaniards into South America, has run
wild over the whole country and has increased in
numbers at an unparalleled rate, I asked myself what
could so recently have exterminated the former horse
under conditions of life apparently so favourable. But

CHAP.

x.

EXTINCTION.

319

how utterly groundless was my astonishment1 Professor Owen soon perceived that the tooth, though solike that of the existing horse, belonged to an extinct
species. Had this horse been still living, but in somedegree rare, no naturalist would have felt the least surprise at its rarity ; for rarity is the attribute of a vast
number of species of all classes, in all countries. If
we ask ourselves why this or that species is rare, we
answer that something is unfavourable in its conditionsof life ; but what that something is, we can hardly evertell. On the supposition of the fossil horse still existing as a rare species, we might have felt certain fromthe analogy of all other mammals, even of the slowbreeding elephant, and from the history of the naturalisation of the domestic horse in South America, that
under more favourable conditions it would in a very few
years have stocked the whole continent. But we could
not have told what the unfavourable conditions werewhich checked its increase, whether some one orseveral contingencies, and at what period of the horse’s
life, and in what degree, they severally acted. If
the conditions had gone on, however slowly, becoming less and less favourable, we assuredly should nothave perceived the fact, yet the fossil horse would certainly have become rarer and rarer, and finally extinct ;-its place being seized on by some more successfulcompetitor.
It is most difi?cult always to remember that theincrease of every living being is constantly being
checked by unperceived injurious agencies ; and thatthese same unperceived agencies are amply suflicient tocause rarity, and finally extinction. We see in manycases in the more recent tertiary formations, that rarity
precedes extinction; and we know that this has been
the progress of events with those animals which have

320

GEOLOGICAL SUCCESSION.

CHAP. X.

been exteiminated, either locally or wholly, throughman’s agency. I may repeat what I published in 1845,
namely, that to admit that species generally becomerare before they become extinct-to feel no surprise at
the rarity of a species, and yet to marvel greatly when
it ceases to exist, is much the same as to admit thatsickness in the individual is the forerunner of deathto feel no surprise at sickness, but when the sick man
dies, to wonder and to suspect that he died by someunkn~~
deedn of violence.
The theory of natural selection is grounded on the
belief that each new variety, and ultimately each new
species, is produced and maintained by having someadvantage over those with which it comes into competition ; and the consequent extinction of less-favoured
forms almost inevitably follows. It is the same with
our domestic productions: when a new and slightly
improved variety has been raised, it at first supplants
the less improved varieties in the same neighbourhood ;when much improved it is transported far and near,like our short-horn cattle, and takes the place of otherbreeds in other countries. Thus the appearance of newforms and the disappearance of old forms, both naturaland artificial, are bound together. I n certain flourishinggroups, the number of new specific forms which havebeen produced within a given time is probably greaterthan that of the old forms which have been exterminated ; but we know that the number of species has notgone on indefinitely increasing, at least during the later
geological periods, so that looking to later times wemay believe that the production of new forms has caused
the extinction of about the same number of old forms.
The competition will generally be most severe, as
formerly explained and illustrated by examples, between
the forms which are most like each other in all respects.

CHAP. X.

EXTINCTION.

321

Hence the improved and modified descendants of a
species will generally cause the extermination of theparent-species ; and if many new forms have been developed from any one species, the nearest allies of thatspecies, i. e. the species of the same genus, will be themost liable to extermination. Thus, as I believe, anumber of new species descended from one species, thatis a new genus, comes to supplant an old genus, belonging to the same family. But it must often have happened
that a new species belonging to some one group will haveseized on the place occupied by a species belonging toa distinct group, and thus caused its extermination;and if many allied forms be developed from the successful intruder, many will have to yield their places ; andit w i l l generally be allied forms, which will sufferfrom some inherited inferiority in common. But whether it be species belonging to the same or to a
distinct class, which yield their places to other species
which have been modified and improved, a few of thesufferers may often long be preserved, from being
fitted to some peculiar line of life, or from inhabitingsome distant and isolated station, where they haveescaped severe competition. For instance, a single
species of Trigonia, a great genus of shells in thesecondary formations, survives in the Australian seas ;and a few members of the great and almost extinctgroup of Ganoid fishes still inhabit our fresh waters.
Therefore the utter extinction of a group is generally, as we have seen, a slower process than its production.
With respect to the apparently sudden exterminationof whole families or orders, as of Trilobites at the closeof the palaeozoic period and of Ammonites at the closeof the secondary period, we must remember what has
been already said on the probable wide intervals of time322

GEOLOGICAL SUCCESSIOX,

CHAP. 8.

between our consecutive formations ; and in these intervals there may have been much slow ext,ermination.
Moreover, when by sudden immigration or by unusually
rapid development, many species of a new group havetaken possession of a new area, t h y will have exterminated in a correspondingly rapid manner many of theold inhabitants; and the forms which thus yield theirplaces will commonly be allied, for they will partake ofsome inferiority in common.
Thus, as it seems to me, the manner in which single
species and whole groups of species become extinct,
accords well with the theory of natural selection. We
need not marvel at extinction ; if we must marvel, let
it be at our presumption in imagining for a momentthat we understand the many complex contingencies,on which the existence of each species depends. If weforget for an instant, that each species tends to increase
inordinately, and that some check is always in action,
yet seldom perceived by us, the whole economy ofnature will be utterly obscured. Whenever we canprecisely say why this species is more abundant in individuals than that ; why this species and not anothercan be naturalised in a given country; then, and nottill then, we may justly feel surprise why we cannot
account for the extinction of this particular species orgroup of species.

On the Form of L;fe changifig almost simultaneously
throughout the World-Scarcely
any palaeontologicaldiscovery is more striking than the fact, that the formsof life change almost simultaneously throughout theworld. Thus our European Chalk formation can berecognised in many distant parts of the world, underthe most different climates, where not a fragment of themineral chalk itself can be found; namely, in North

CHAP. X.

THROUGHOUT THE WORLD.

323

America, in equatorial South America, in Tierra delFuego, at the Cape of Good Hope, and in the peninsulaof India. For at these distant points, the organic remains in certain beds present an unmistakeable degreeof resemblance to those of the Chalk. It is not thatthe same species are met with; for in some cases not
one species is identically the same, but they belong to
the same families, genera, and sections of genera, andsometimes are similarly characterised in such trifling
points as mere superficial sculpture. Moreover otherforms, which are not found in the Chalk of Europe, butwhich occur in the formations either above or below, aresimilarly absent at these distant points of the world. I n
the several successive palaeozoic formations of Russia,
Western Europe and North America, a similar parallelism in the forms of life has been observed by several
authors: so it is, according to Lyell, with the several
European and North American tertiary deposits. Even
if the few fossil species which are common to the Old
and New Worlds be kept wholly out of view, the generalparallelism in the successive forms of life, in the stagesof the widely separated palaeozoic and tertiary periods,
would still be manifest, and the several formations
could be easily correlated.
These observations, however, relate to the marine inhabitants of distant parts of the world : we have notsufficient data to judge whether the productions of theland and of fresh water change at distant points in thesame parallel manner. We may doubt whether theyhaye thus changed : if the Megatherium, Mylodon,
Macrauchenia, and Toxodon had been brought to Europe
from La Plata, without any information in regard totheir geological position, no one would have suspectedthat they had coexisted with still living sea-shells ; but
as these anomalous monsters coexisted with the Masto-

324

GEOLOGICAL SUCCESSION,

CHAP.

x.

don and Horse, it might at least have been inferred thatthey had lived during one of the later tertiary stages.
When the marine forms of life are spoken of as
having changed simultaneously throughout the world,
it must not be supposed that this expression relates tothe same thousandth or hundred-thousandth year, oreven that it has a very strict geological sense; for if
all the marine animals which live at the present day inEurope, and all those that lived in Europe during the
Pleistocene period (an enormously remote period asmeasured by years, including the whole glacial epoch),mere to be compared with those now living in SouthAmerica or in Australia, the most skilful naturalist
would hardly be able to say whether the existing or thepleistocene inhabitants of Europe resembled most closely
those of the southern hemisphere. So, again, several
highly competent observers believe that the existingproductions of the United States are more closely related
to those which lived in Europe during certain later tertiary stages, than to those which now live here; andif this be so, it is evident that fossiliferous beds deposited at the present day on the shores of North
America would hereafter be liable t o be classed withsomewhat older European beds. Nevertheless, looking
to a remotely future epoch, there can, I think, be little
doubt that all the more modern marine formations,
namely, the upper Pliocene, the Pleistocene and strictly
modern beds, of Europe, North and South America, andAustralia, from containing fossil remains in some degree
allied, and from not including those forms which are
only found in the older underlying deposits, would becorrectly ranked as simultaneous in a geological sense.
The fact of the forms of life changing simultaneously,in the above large sense, at distant parts of the world,h s greatly struck those admirable observers, MM.

CHAP.

x.

THROUGHOUT THE WORLD.

325

de Verneuil and d‘hchiac. After referring to theparallelism of the pala?ozoic forms of life in various
parts of Europe, they add, “If struck by this strange
sequence, we turn our attention to North America, andthere discover a series of analogous phenomena, it willappear certain that all these modifications of species,
their extinction, and the introduction of new ones, cannot be owing to mere changes in marine currents orother causes more or less local and temporary, but depend on general laws which govern the whole animal
kingdom.” 31. Barrande has made forcible remarks toprecisely the same effect. It is, indeed, quite futile tolook to changes of currents, climate, or other physicalconditions, as the cause of these great mutations in theforms of life throughout the world, under the most different climates. We must, as Barrande has remarked,
look to some speciallaw. We shall see this more clearly
when we treat of the present distribution of organic
beings, and find how slight is the relation between thephysical conditions of various countries, and the natureof their inhabitants.
This great fact of the parallel succession of the formsof life throughout the world, is explicable on the theoryof natural selection. New species are formed by new
varieties arising, which have some advantage overolder forms ; and those forms, which are already dominant, or have some advantage over the other forms intheir own country, would naturally oftenest give rise to
new varieties or incipient species ; for these latter must
be victorious in a still higher degree in order to be preserved and to survive. We have distinct evidence on
this head, in the plants which are dominant, that is,which are commonest in their own homes, and are mostwidely diffused, having produced the greatest numberof new varieties. It is also natural that the domi-

326

GEOLOGICAL SUCCESSION,

CHAP. X.

nant, varying, and far-spreading species, which alreadyhave invaded to a certain extent the territories of otherspecies, should be those which would have the best
chance of spreading still further, and of giving rise in
new countries to new varieties and species. The processof diffusion may often be very slow, being dependent
on climatal and geographical changes, or on strange
accidents, but in the long run the dominant forms will
generally succeed in spreading. The diffusion would, it
is probable, be slower with the terrestrial inhabitants ofdistinct continents than with the marine inhabitants ofthe continuous sea. JTTemight therefore expect to find,
as we apparently do find, a less strict degree of parallel
succession in the productions of the land than of the sea.
Dominant species spreading from any region might
encounter still more dominant species, and then theirtriumphant course, or even their existence, would cease.
We know not at all precisely what are all the conditions
most favourable for the multiplication of new and dominant species; but we can, I think, clearly see that a
number of individuals, from giving a better chance of
the appearance of favourable variations, and that severe
competition with many already existing forms, would behighly favourable, as would be the power of spreading
into new territories. A certain amount of isolation,
recurring at long intervals of time, would probably bealso favourable, as before explained. One quarter ofthe world may have been most favourable for the production of new and dominant species on the land, andanother for those in the waters of the sea. If two great
regions had been for a long period favourably circumstanced in an equal degree, whenever their inhabitantsmet, the battle would be prolonged and severe; andsome from one birthplace and some from the othermight be victorious. But in the course of time, theC&W. x.

THROUGHOUT THE WORLD.

327

forms dominant in the highest degree, wherever produced, would tend everywhere to prevail. As they prevailed, they would cause the extinction of other andinferior forms; and as these inferior forms would beallied in groups by inheritance, whole groups would
tend slowly to disappear; though here and there asingle member might long be enabled to survive.
Thus, as it seems to me, the parallel, and, taken in alarge sense, simultaneous, succession of the same formsof life throughout the world, accords well with the principle of new species having been formed by dominant
species spreading widely and varying ; the new species
thus produced being themselves dominant owing to inheritance, and to having already had some advantage
over their parents or over other species; these again
spreading, varying, and producing new species. The
forms which are beaten and which yield their places tothe new and victorious forms, will generally be allied ingroups, from inheriting some inferiority in common ;and therefore as new and improved groups spreadthroughout the world, old groups will disappear fromthe world; and the succession of forms in both ways
will everywhere tend to correspond.
There is one other remark connected with this subject
worth making. I have given my reasons for believing that all our greater fossiliferous formations were
deposited during periods of subsidence ; and thatblank intervals of vast duration occurred during theperiods when the bed of the sea was either stationary or rising, and likewise when sediment was not
thrown down quickly enough to embed and preserve
organic remains. During these long and blank intervals I suppose that the inhabitants of each region
underwent a considerable amount of modification andextinction, and that there was much migration from

328

GEOLOGICAL SUCCESSION.

CHAP.

x.

other parts of the world. As we have reason tobelieve that large areas are affected by the same movement, it is probable that strictly contemporaneous formations have often been accumulated over very wide
spaces in the same quarter of the world; but we arefar from having any right to conclude that this has invariably been the case, and that large areas have invariably been affected by the same movements. When two
formations have been deposited in two regions during
nearly, but not exactly the same period, we should findin both, from the causes explained in the foregoingparagraphs, the same general succession in the forms oflife ; but the species would not exactly correspond ; forthere will have been a little more time in the one region
than in the other for modification, extinction, and immigration.
I suspect that cases of this nature have occurred inEurope. Mr. Prestwich, in his admirable Memoirs on
the eoeene deposits of England and France, is able todraw a close general parallelism between the successive
stages in the two countries; but when he comparescertain stages in England with those in France, although
he finds in both a curious accordance in the numbersof the species belonging to the same genera, yet the
species themselves differ in a manner very difficult toaccount for, considering the proximity of the two areas,-unless, indeed, it be assumed that an isthmus separated
two seas inhabited by distinct, but contemporaneous,faunas. Lye11 has made similar observations on some of
the later tertiary formations. Barrande, also, shows that
there is a striking general parallelism in the successive
Silurian deposits of' Bohemia and Scandinavia ; nevertheless he finds a surprising amount of difference inthe species. If the several formations in these regions have not been deposited during the same exact

CHAP. X.

AFFINITIES O F EXTINCT SPECIES.

329

periods,-a formation in one region often corresponding
with a blank interval in the other,-and if in bothregions the species have gone on slowly changing
during the accumulation of the several formations andduring the long intervals of time between them; inthis case, the several formations in the two regions
could be arranged in the same order, in accordancewith the general succession of the form of life, and theorder would falsely appear to be strictly parallel ; nevertheless the species would not all be the same in theapparently corresponding stages in the two regions.
On the Afinities of extinct Species to each other, and toliving forms.-Let
us now look to the mutual affinities
of extinct and living species. They all fa.ll into one
grand natural system ; and this fact is at once explainedon the principle of descent. The more ancient any
form is, the more, as a general rule, it differs from living
forms. But, as Buckland long ago remarked, all fossils
can be classed either in still existlinggroups, or between
them. That the extinct forms of life help to fill up thewide intervals between existing genera, families, andorders, cannot be disputed. For if we confine our attention either to the living or to the extinct alone, theseries is far less perfect than if we combine both into
one general system. With respect to the Vei-tebrata,
whole pages could be filled with striking illustrations
from our great palaeontologist, Owen, showing how extinct animals fall in between existing groups. Cuvier
ranked the Iluminants and Pachyderms, as the two mostdistinct orders of' mammals ; but Owen has discoveredso many fossil links, that he has had to alter the wholeclassification of these two orders ; and has placed certain
pachyderms in the same sub-order with ruminants : forexample, he dissolves by fine gradations the apparently

330

GEOLOGICAL SUCCESSION.

CHAP. X.

wide difference between the pig and the camel. In
regard to the Invertebrata, Barrande, and a higher
authority could not be named, asserts that he is every
day taught that palzozoic animals, though belonging to
the same orders, families, or genera with those living
at the present day, were not at this early epoch limited
in such distinct groups as they now are.
Some writers have objected to any extinct species orgroup of species being considered as intermediate between living species or groups. If by this term it ismeant that an extinct form is directly intermediate in
all its characters between two living forms, the objection is probably valid. But I apprehend that in aperfectly natural classification many fossil species would
have to stand between living species, and some extinct
genera between living genera, even between genera
belonging to distinct families. The most common case,especially with respect to very distinct groups, such as
fish and reptiles, seems to be, that supposing them to be
distinguished at the present day from each other by adozen characters, the ancient members of the same twogroups would be distinguished by a somewhat lesser
number of characters, so that the two groups, thoughformerly quite distinct, at that period made some small
approach to each other.
It is a common belief that the more ancient a formis, by so much the more it tends to connect by some ofits characters groups now widely separated from eachother. This remark no doubt must be restricted tothose groups which have undergone much change in thecourse of geological ages; and it would be difficult toprove the truth of the proposition, for every now andthen even a living animal, as the Lepidosiren, is discovered having affinities directed towards very distinctgroups. Yet if we compare the older Reptiles andCHAP. X.

AFFINITIES OF EXTINCT SPECIES.

331

Batrachians, the older Fish, the older Cephalopods, andthe eocene Mammals, with the more recent membersof the same classes, we must admit that there is sometruth in the remark.
Let us see how far these several facts and inferences
accord with the theory of descent with modification. As
the subject is somewhat complex, I must request thereader to turn to the diagram in the fourth chapter.
We may suppose that the numbered letters represent
genera, and the dotted lines diverging from themthe species in each genus. The diagram is much too
simple, too few genera and too few species being
given, but this is unimportant for us. The horizontal
lines may represent successive geological formations,and all the forms beneath the uppermost line may
be considered as extinct. The three existing genera,U’~,
qI4, pI4, will form a small family; ZI l4 and f 1 4 aclosely allied family or sub-family ; and o l4 el4, m 14, a
third family. These three families, together with themany extinct genera on the several lines of descentdiverging from the parent-form A, will form an order ;for all will have inherited something in common
from their ancient and common progenitor. On theprinciple of the continued tendency to divergence ofcharacter, which was formerly illustrated by this diagram, the more recent any form is, the more it will
generally differ from its ancient progenitor. Hence wecon understand the rule that the most ancient fossils
differ most from existing forms. W e must not, however, assume that divergence of character is a necessary
contingency ; it depends solely on the descendants froma species being thus enabled to seize on many and different places in the economy of nature. Therefore it isquite possible, as we have seen in the case of some
Silurian forms, that a species might go on being slightly

332

GEOLOGICAL SUCCESSION.

CHAP. x.

modified in relation to its slightly altered conditions oflife, and yet retain throughout a vast period the samegeneral characteristics. This is represented in the diagram by the letter F 14.
All the many forms, extinct and recent, descended
from A, make, as before remarked, one order ; and thisorder, from the continued effects of extinction anddivergence of character, has become divided into several sub-families and families, some of which aresupposed to have perished at different periods, and someto have endured to the present day.
By looking at the diagram we can see that if manyof the extinct forms, supposed to be embedded in the
successive formations, were discovered at several points
low down in the series, the three existing families on theuppermost line would be rendered less distinct from eachother. If, for instance, the genera al, a5,al0,fs7m3,m6,m9,
were disinterred, these three families would be so closely
linked together that they probably would have to beunited into one great family, in nearly the same manneras has occurred with ruminants and pachyderms. Yet
he who objected to call the extinct genera, which thus
linked the living genera of three families together, intermediate in character, would be justified, as they areintermediate, not directly, but only by a long and circuitous course through many widely different forms. Ifmany extinct forms were to be discovered above oneof the middle horizontal lines or geological formations-for instance, above No. V1.-but none from beneath
this line, then only the two families on the left hand(namely, aI4,&c., and b14, &c.) would have to be united
into one family; and the two other families (namely,
to f'" now including five genera, and 014to m14)would
yet remain distinct. These two families, however, would
be less distinct from each other than they were before theCHAP.S.

AFFINITIES OF EXTINCT SPECIES.

333

discovery of the fossils. If, for instance, we suppose theexisting genera of the two families to differ from eachother by a dozen characters, in this case the genera, atthe early period marked VI., would differ by a lesser
number of characters ; for at this early stage of descent
they have not diverged in character from the common
progenitor of the order, nearly SO much as they subsequently diverged. Thus it comes that ancient andextinct genera are often in some slight degree intermediate in character between their modified descendants,
or between their collateral relations.
In nature the case w i l l be far move complicated thanis represented in the diagram ; for the groups will havebeen more numerous, they will have endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last
volume of the geological record, and that in a very broken
condition, we have no right to expect, except in veryrare cases, to fill up wide intervals in the natural system,and thus unite distinct families or orders. All that
we have a right to expect, is that those groups, whichhave within known geological periods undergone muchmodification, should in the older formations make someslight approach to each other; so that the older members should differ less from each other in some of theircharacters than do the existing members of the samegroups ; and this by the concurrent evidence of our best
palteeontologists seems frequently to be the case.
Thus, on the theory of descent with modification, themain facts with respect to the mutual affinities of theextinct foims of life to each other and to living forms,
seem to me explained in a satisfactory manner. And
they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of
any great period in the earth's history will be inter-

334

GEOLOGICAL SUCCESSION.

CHAP. X.

mediate in general character between that which preceded and that which succeeded it. Thus, the species
which lived at the sixth great stage of descent in the
diagram are the modified offspring of those which lived
at the fifth stage, and are the parents of those whichbecame still more modified at, the seventh stage ; hence
they could hardly fail to be nearly intermediate incharacter between the forms of life above and below.
We must, however, allow for the entire extinction ofsome preceding forms, and for the coming in of quite
new forms by immigratioh, and for a large amount ofmodification, during the long and blank intervals between the successive formations. Subject to these allowances, the fauna of each geological period undoubtedly
is intermediate in character, between the preceding andsucceeding faunas. I need give only one instance,
namely, the manner in which the fossils of the Devoniansystem, when this system was first discovered, were atonce recognised by palaeontologists as intermediate incharacter between those of the overlying carboniferous, and underlying Silurian system. But eachfauna is not necessarily exactly intermediate, as
unequal intervals of time have elapsed between consecutive formations.
It is no real objection to the truth of the statement,that the fauna of each period as a whole is nearly intermediate in character between the preceding andsucceeding faunas, that certain genera offer exceptions
to the rule. For instance, mastodons and elephants,
when arranged by Dr. Falconer in two series, firstaccording to their mutual affinities and then according
to their periods of existence, do not accord in arrangement. The species extreme in character are not theoldest, or the most, recent; nor are those which areintermediate in character, intermediate in age. But

CHAP.X.

AFFINITIES OF EXTINCT SPECIES.

335

supposing for an instant, in this and other such cases,that the record of the first appearance and disappearanceof the species was perfect, we have no reason to believethat forms successively produced necessarily endure forcorresponding lengths of time: a very ancient form
might occasionally last much longer than a form elsewhere subsequently produced, especially in the case ofterrestrial productions inhabiting separated districts.
To compare small things with great : if the principal
living and extinct races of the domestic pigeon werearranged as well as they could be in serial affinity, thisarrangement would not closely accord with the order intime of their production, and still less with the order oft,lieir disappearance ; for the parent rock-pigeon now
lives ; and many varieties between the rock-pigeon andthe carrier have become extinct; and carriers which
are extreme in the important character of length ofbeak originated earlier than short-beaked tumblers,
which are at the opposite end of the series in this same
respect.
Closely connwted with the statement, that the organic remains from an intermediate formation are insome degree intermediate in character, is the fact,
insisted on by all palaeontologists, that fossils from twoconsecutive formations are far more closely related toeach other, than are the fossils from two remote formations. Pictet gives as a well-known instance, thegeneral resemblance of the organic remains from theseveral stages of the chalk formation, tliough the species
are distinct in each stage. This fact alone, from its
generality, seems to have shaken Professor Pictet in his
firm belief in the immutability of species. H e who isacquainted with the distribution of existing species overthe globe, will not attempt to account for the close
resemblance of the distinct species in closely consecutive

336

GEOLOGICAL SUCCESSION.

CHAP.

x.

formations, by the physical conditions of the ancientareas having remained nearly the same. Let it beremembered that the forms of life, at least those inhabiting the sea, have changed almnst simultaneously
throughout the world, and therefore under the mostdifferent climates and conditions. Consider the prodigious vicissitudes of climate during the pleistocene
period, which includes the whole glacial period, andnote how little the specific forms of the inhabitants ofthe sea have been affected.
On the theory of descent, the full meaning of thefact of fossil remains from closely consecutive formations, though ranked as distinct species, being closely
related, is obvious. As the accumulation of each formation has often been interrupted, and as long blankintervals have intervened between successive formations,
we ought not to expect to find, as I attempted to show
in the last chapter, in any one or two formations all theintermediate varieties between the species which appeared at the commencement and close of these periods ;but we ought to find after intervals, very long as
measured by years, but only moderately long as
measured geologically, closely allied forms, or, as they
have been called by some authors, representative species; and these we assuredly do find. We find, inshort, such evidence of the slow and scarcely sensible
mutation of specific forms, as we have a just right toexpect to find.

On the state of Development of Ancient Perms.-There
has been much discussion whether recent forms are
more highly developed than ancient. I will not here
enter on this subject, for naturalists have not as yet
defined to each other’s satisfaction what is meant byhigh and low forms. But in one particular sense theCHAP. X.

STATE OF DEVELOPMENT.

337

more recent forms must, on my theory, be higher thanthe more ancient; for each new species is formed byhaving had some advantage in the struggle for life
over other and preceding forms. If under a nearly
similar climate, the eocene inhabitants of one quarterof the world were put into competition with the existing inhabitants of the same or some other quarter,
the eocene fauna or flora would certainly be beaten
and exterminated; as would a secondary fauna by aneocene, and a palaeozoic fauna by a secondary fauna.
I do not doubt that this process of improvement has
affected in a marked and sensible manner the organisation of the more recent and victorious forms of life,
in comparison with the ancient and beaten forms ; but
I can see no way of testing this sort of progress.
Crustaceans, for instance, not the highest in their own
class, may have beaten the highest molluscs. Fromthe extraordinary manner in which European productions have recently spread over New Zealand, andhave seized on places which must have been previously
occupied, we may believe, if all the animals and plantsof Great Britain were set free in New Zealand, that in
the course of time a multitude of British forms would
become thoroughly naturalized there, and would exterminate many of the natives. On the other hand,from what we see now occurring in New Zealand, andfrom hardly a single inhabitant of the southern hemisphere having become wild in any part of Europe,
we may doubt, if all the productions of New Zealandwere set free in Great Britain, whether any considerable number would be enabled to seize on places now
occupied by our native plants and animals. Under thispoint of view, the productions of Great Britain may besaid to be higher than those of New Zealand. Yet
the most skilful naturalist from an examination of the338

GEOLOGICAL SUCCESSION.

CIIAP.

x.

species of the two countries could not have foreseen
this result.
Agassiz insists that ancient animals resemble to acertain extent the embryos of recent animaIs of thesame classes; or that the geological succession of
extinct forms is in some degree parallel to the embryological development of recent forms. I must follow
Pictet and Huxley in thinking that the truth of this
doctrine is very far from proved. Yet I fully expect tosee it hereafter confirmed, at least in regard to subordinate groups, which have branched off from each otherwithin comparatively recent times. For this doctrineof Agassiz accords well with the theory of natural
selection. I n a future chapter I shall attempt to showthat t,he adult differs from its embryo, owing to variations supervening at a not early age, and being inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered, continually adds;inthe course of successive generations, more and moredifference to the adult.
Thus the embryo comes to be left as a sort of picture,preserved by nature, of the ancient and less modified
condition of each animal. This view may be true, andyet it may never be capable of full proof. Seeing, forinstance, that the oldest known mammals, reptiles, andfish strictly belong to their own proper classes, thoughsome of these old forms are in a slight degree less distinct from each other than are the typical members of
the same groups at the present day, it would be vain tolook for animals having the common embryologicalcharacter of the Vertebrata, until beds far beneath thelowest Silurian strata are discovered- a discovery ofwhich the chance is very small.

On the Succession of the ~ a m eTypes within the same

CHAP. X.

SANE TYPES IN SAME AREAS.

339

areas, during the later tertiary periods.-Mr. Clift many
years ago showed that the fossil mammals from theAustralian caves were closely allied to the living marsupials of that continent. In South America, a similar
relationship is manifest, even to an uneducated eye,in the gigantic pieces of armour like those of the armadillo, found in several parts of La Plata ; and Professor
Owen has shown in the most striking manner that mostof the fossil mammals, buried there in such numbers,
are related to South American types. This relationship is even more clearly seen in the wonderful collection of fossil bones made by MM. Lund and Clausen in
the caves of Brazil. I was so much impressed withthese facts that I strongly insisted, in 1839 and 1845,
on this “law of the succession of types,”-on “this wonderful relationship in the same continent between thedead and the living.” Professor Owen has subsequently
extended the same generalisation to the mammals of
the Old World. We see the same law in this author’s
restorations of the extinct and gigantic birds of New
Zealand. We see it also in the birds of the caves ofBrazil. Mr. Woodward has shown that the same law
holds good with sea-shells, but from the wide distribution of most genera of molluscs, it is not well displayedby them. Other cases could be added, as the relation
between the extinct and living land-shells of Madeira ;and between the extinct and living brackish-water shellsof the Aralo-Caspian Sea.
Now what does this remarkable law of the succession of the same types within the same areas niean?
He would be a bold man, who after comparing the present climate of Australia and of parts of South America
under the same latitude, would attempt to account, on
the one hand, by dissimilar physical conditions for thedissimilarity of the inhabitants of these two continents,

340

GEOLOGICAL SUCCESSION.

CHAP.

x.

and, on the other hand, by similarity of conditions, for
the uniformity of the same types in each during thelater tertiary periods. Nor can it be pretended that it
is an immutable law that marsupials should have been
chiefly or solely produced in Australia ; or that Edentata and other American types should have been solely
produced in South America, For we know that Europe
in ancient times was peopled by numerous marsupials ;and I have shown in the publications above alluded to,that in America the law of distribution of terrestrial
mammals was formerly different from what it now is.
North America formerly partook strongly of the present character of the southern half of the continent;
and the southern half was formerly more closely allied,than it is at present, to the northern half. I n a similarmanner we know from Falconer and Cautley’s discoveries, that northern India was formerly more closely
related in its mammals to Africa than it is at the present time. Analogous facts could be given in relation
to the distribution of marine animals.
On the theory of descent with modification, the great
law of the long enduring, but not immutable, successionof the same types within the same areas, is at once
explained; for the inhabitants of each quarter of theworld will obviously tend to leave in that quarter,
during the next succeeding period of time, closely
allied though in some degree modified descendants. If
the inhabitants of one continent formerly differed
greatly from those of another continent, so will theirmodified descendants still differ in nearly the samemanner and degree. But after very long intervals oftime and after great geographical changes, permittingmuch inter-migration, the feebler will yield to themore dominant forms, and there will be nothing immutable in the laws of past and present distribution.

CHAP.

x.

SAME TYPES IN SAME AREAS.

311

It may be asked in ridicule, whether I suppose thatthe megatherium and other allied huge monsters haveleft behind them in South America the sloth, armadillo,
and anteater, as their degenerate descendants. Thiscannot for an instant be admitted. These huge animals have become wholly extinct, and have left no progeny. But in the caves of Brazil, there are many
extinct species which are closely allied in size and inother characters to the species still living in SouthAmerica; and some of these fossils may be the actual
progenitors of living species. It must not be forgotten that, on my theory, all the species of the same
genus have descended from some one species; so that
if six genera, each having eight species, be found in one
geological formation, and in the next succeeding formation there be six other allied or representative generawith the same number of species, then we may conclude that only one species of each of the six older
genera has left modified descendants, constituting thesix new genera. The other seven species of the oldgenera have all died out and have left no progeny. Or,
which would probably be a far commoner case, two or
three species of two or three alone of the six older
genera will have been the parents of the six new
genera; the other old species and the other wholegenera having become utterly extinct. I n failingorders, with the genera and species decreasing in
numbers, as apparently is the case of the Edentata ofSouth America, still fewer genera and species will haveleft modified blood-descendants.
Summary of t h preceding and pre8ent C’hapters.--I
have attempted to show that the geological record is
extremely imperfect ; that only a small portion of the
globe has been geologically explored with care; that

342

GEOLOGICAL SUCCESSION.

CHAP. X .

only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of
specimens and of species, preserved in our museums, isabsolutely as nothing compared with the incalculable
number of generations which must have passed awayeven during a single formation; that, owing to sibsidence being necessary for the accumulation of fossiliferous deposits thick enough to resist future degradation,
enormous intervals of time have elapsed between the
Ruccessive formations ; that there has probably been
more extinction during the periods of subsidence, andmore variation during the periods of elevation, andduring the latter the record will have been least perfectly kept; that each single formation has not been
continuously deposited ; that the duration of eachformation is, perhaps, short compared with the average
duration of specific forms; that migration has played
an important part in the first appearance of new forms
in any one area and formation; that widely ranging
species are those which have varied most, and haveoftenest given rise to new species ; and that varieties
have a t first often been local. All these causes taken
conjointly, must have tended to make the geologicalrecord extremely imperfect, and will to a large extent
explain why we do not find interminable varieties, connecting together all the extinct and existing forms oflife by the finest graduated steps.
He who rejects these views on the nature of thegeological record, will rightly reject my whole theory.
For he may ask in vain where are the numberlesstransitional links which must formerly have connected
the closely allied or representative species, found in
the several stages of the same great formation. H emay disbelieve in the enormous intervals of time whichhave elapsed be tween our consecutive formations ; heCHAP.

x.

SUMMARY.

343

may overlook how important a part migration musthave played, Then the formations of any one great
region alone, as that of Europe, are considered ; he mayurge the apparent, but often falsely apparent, sudden
coming in of whole groups of species. He may ask
where are the remains of those infinitely numerousorganisms which must have existed long before thefirst bed of the Silurian system was deposited: I cananswer this latter question only hypothetically, by saying that as far as we can see, where our oceans now
extend they have for an enormous period extended, andwhere our oscillating continents now stand they havestood ever since the Silurian epoch; but that long
before that period, the world may have presented awholly different aspect ; and that the older continents,
formed of formations older than any known to us, may
now all be in a metamorphosed condition, or may lie
buried under the ocean.
Passing from these difficulties, all the other great
leading facts in palaeontology seem to me simply tofollow on the theory of descent with modification
through natural selection. We can thus understandhow it is that new species come in slowly and successively; how species of different classes do not necessarily change together, or at the same rate, or in thesame degree; yet in the long run that all undergo
modification to some extent. The extinction of oldforms is the almost inevitable consequence of the production of new forms. We can understand why whena species has once disappeared it never reappears.
Groups of species increase in numbers slowly, andendure for unequal periods of time ; for the process ofmodification is necessarily slow, and depends on many
complex contingencies. The dominant species of thelarger dominant groups tend to leave many modified

344

GEOLOGICAL SUCCESSION.

CHAP. X.

descendants, and thus new sub-groups and groups areformed. As these are formed, the species of the lessvigorous groups, from their inferiority inherited from acommon progenitor, tend to become extinct together, andto leave no modified offspring on the face of the earth.
But the utter extinction of a whole group of species
may often be a very slow process, from the survival of afew descendants, lingering in protected and isolated
situations. When a group has once wholly disappeared,
it does not reappear; for the link of generation has
been broken.
We can understand how the spreading of the dominant forms of life, which are those that oftenest vary,
will in the long run tend to people the world withallied, but modified, descendants ; and these will generally succeed in taking the places of those groups ofspecies which are their inferiors in the struggle forexistence. Hence, after long intervals of time, theproductions of the world will appear to have changed
simultaneously.
We can understand how it is that all the forms oflife, ancient and recent, make together one grandsystem ; for all are connected by generation. We canunderstand, from the continued tendency to divergenceof character, why the more ancient a form is, the more
it generally differs from those now living. Why ancientand extinct forms often tend to fill up gaps between
existing forms, sometimes blending two groups previously classed as distinct into one ; but more commonly
only bringing them a little closer together. The moreancient a form is, the more often, apparently, it displays characters in some degree intermediate between
groups now distinct; for the more ancient a form is,
the more nearly it will be related to, and consequently
resemble, the common progenitor of groups, Pinee be-CHAP. X.

SUMMARY.

345

come widely divergent. Extinct forms are seldom
directly intermediate between existing forms ; but areintermediate only by a long and circuitous course
through many extinct and very different forms. We
can clearly see why the organic remains of closely consecutive formations are more closely allied to eachother, than are those of remote formations; for theforms are more closely linked together by generation :we can clearly see why the remains of an intermediate
formation are intermediate in character.
The inhabitants of each successive period in theworld's history have beaten their predecessors in therace for life, and are, in so far, higher in the scale ofnature ; and this may account for that vague yet illdefined sentiment, felt by many palaeontologists, thatorganisation on the whole has progressed. If it should
hereafter be proved that ancient animals resemble toa certain extent the embryos of more recent animalsof the same class, the fact will be intelligible. The
succession of the same types of structure within thesame areas during the later geological periods ceases tobe mysterious, and is simply explained by inheritance.
If then the geological record be as imperfect as I
believe it to be, and it may a t least be asserted thatthe record cannot be proved to be much more perfect,
the main objections to the theory of natural selection
are greatly diminished or disappear. On the other
hand, all the chief laws of palzontology plainly proclaim, as it seems to me, that species have been produced by ordinary generation: old forms having been
supplanted by new and improved forms of life, producedby the laws of variation still acting round us, and preserved by Natural selection.

346

GEOGRAPHICAL DISTRIBUTION.

C H 9P T E R

CHAP. XI.

x I.

GEOGRAPHICAL
DISTRIBUTIOX.
Present distribution cannot be accounted for by differences in physical conditions - lmportance of barriers - Affinity of the productions of the same continent - Centres of creation - Meansof dispersal, by changes of climate and of the level of the land,and by occasional means - Dispersal during the Glacial period
co-extensive with the world.

INconsidering the distribution of organic beings overthe face of the globe, the first great fact which strikes
us is, that neither the similarity nor the dissimilarityof the inhabitants of various regions can be accounted
for by their climatal and other physical conditions. Of
late, almost every author who has studied the subject
has come to this conclusion. The case of America
alone would almost suffice to prove its truth: for if we
exclude the northern parts where the circumpolar landis almost continuous, all authors agree that one of themost fundamental divisions in geographical distribution
is that between the New and Old Worlds; yet if wetravel over the vast American continent, from thecentral parts of the United States to its extreme
southern point, we meet with the most diversified conditions; the most humid districts, arid deserts, lofty
mountains, grassy plains, forests, marshes, lakes, andgreat rivers, under almost every temperature. There ishardly a climate or condition in the Old World whichcannot be paralleled in the New-at least as closely
as the same species generally require ; for it is a mostrare case to find a group of organisms confined to any
small spot, having conditions peculiar in only a slight

CHAP. XI.

GEOGRAPHICAL DISTRIBUTION.

347

degree; for instance, small areas in the Old WorId
could be pointed out hotter than any in the New
World, yet these are not inhabited by a peculiar fauna
or flora. Notwithstanding this parallelism in the conditions of the Old and New Worlds, how widely different are their living productions !
I n the southern hemisphere, if we compare large
tracts of land in Australia, South Africa, and westernSouth America, between latitudes 25" and 35", we shallfind parts extremely similar in all their conditions, yet
it would not be possible to point out three faunas andfloras more utterly dissimilar. Or again we may compare the productions of South America south of lat.
35Owith those north of 2 5 O , which consequently inhabit
a considerably different climate, and they will be foundincomparably more closely related to each other, thanthey are to the productions of Australia or Africa undernearly the same climate. Analogous facts could begiven with respect to the inhabitants of the sea.
A second great fact which strikes us in our generalreview is, that barriers of any kind, or obstacles to freemigration, are related in a close and important mannerto the differences between the productions of various
regions. We see this in the great difference of nearly
all the terrestrial productions of the New and Old
Worlds, excepting in the northern parts, where theland almost joins, and where, under a slightly different
climate, there might have been free migration for thenorthern temperate forms, as there now is for thestrictly arctic productions. We see the same fact in
the great difference between the inhabitants of Australia, Africa, and South America under the same latitude : for these countries are almost as much isolated
from each other as is possible. On each continent,
also, we see the same fact ; for on the opposite sides of348

GEOGRAPHICAL DISTRIBUTION.

CHAP. XI.

lofty and continuous mountain-ranges, and of great
deserts, and sometimes even of large rivers, we finddifferent productions ; though as mountain-chains,deserts, &c., are not as impassable, or likely to haveendured so long as the oceans separating continents,
the differences are very inferior in degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. KOtwo
marine faunas are more distinct, with hardly a fish,shell, or crab in common, than those of the eastern andwestern shores of South and Central America; yetthese great faunas are separated only by the narrow,
but impassable, isthmus of Panama. Westward of theshores of America, a wide space of open ocean extends,
with not an island as a halting-place for emigrants;
here we have a barrier of another kind, and as soon as
this is passed we meet in the eastern islands of the
Pacific, with another and totally distinct fauna. Sothat here three marine faunas range far northward andsouthward, in parallel lines not far from each other,under corresponding climates ; but from being separated from each other by impassable barriers, eitherof land or open sen, they are wholly distinct. On theother hand, proceeding still further westward from theeastern islands of the tropical parts of the Pacific, we
encounter no impassable barriers, and we have innumerable islands as halting-places, until after travellingover a hemisphere we come to the shores of Africa;and over this vast space we meet with no well-definedand distinct marine faunas. Although hardly one shell,crab or fish is common to the above-named three
approximate faunas of Eastern and Western America
and the eastern Pacific islands, yet many fish range
from the Pacific into the Indian Ocean, and manyshells are common to the eastern islands of the PacificCHAP. XI.

GEOGRAPHICAL DISTRIBUTION.

349

and the eastern shores of Africa, on almost exactly
opposite meridians of longitude.
A third great fact, partly included in the foregoingstatements, is the affinity of the productions of themme continent or sea, though the species themselves
are distinct at different points and stations. It isa law of the widest generality, and every continent
offers innumerable instances. Nevertheless the naturalist in travelling, for instance, from north to south
never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly
related, replace each other. He hears from closely
allied, yet distinct kinds of birds, notes nearly similar,
and sees their nests similarly constructed, but not quite
alike, with eggs coloured in nearly the same manner.
The plains near the Straits of Magellan are inhabitedby one species of Rhea (American ostrich), and northward the plains of La Plata by another species of thesame genus; and not by a true ostrich or emeu, likethose found in Africa and Australia under the same
latitude. On these same plains of La Plata, we see
the agouti and bizcacha, animals having nearly thesame habits as our hares and rabbits and belonging to
the same order of Rodents, but they plainly display
an American type of structure. We ascend the lofty
peaks of the Cordillera and we find an alpine speciesof bizcacha ; we look to the waters, and we do not findthe beaver or musk-rat, but the coypu and capybara,rodents of the American type. Innumerable other instances could be given. If we look to the islands off
the American shore, however much they may differ ingeological structure, the inhabitants, though they may
be all peculiar species, are essentially American. We
may look back to past ages, as shown in the lastchapter, and we find American types then prevalent on

350

GEOGRAPHICAL DlSTRIBUTION.

CHAP. XI.

the American continent and in the American seas.
We see in these facts some deep organic bond, prevailing throughout space and time, over the same areas ofland and water, and independent of their physical conditions. The naturalist must feel little curiosity, who
is not led to inquire what this bond is.
This bond, on my theory, is simply inheritance, thatcause which alone, as far as we positively know, produces organisms quite like, or, as we see in the caseof varieties nearly like each other. The dissimilarity of
the inhabitants of different regions may be attributed
to modification through natural selection, and in a quite
subordinate degree to the direct influence of different
physical conditions. The degree of dissimilarity will depend on the migration of the more dominant forms of life
from one region into another having been effected withmore or less ease, a t periods more or less remote ;-onthe nature and number of the former immigrants ;and on their action and reaction, in their mutual
struggles for life ;-the relation of organism to organism
being, as I have already often remarked, the most important of all relations. Thus the high importance ofbarriers comes into play by checking migration; as
does time for the slow process of modification throughnatural selection. Widely-ranging species, abounding
in individuals, which have already triumphed over many
competitors in their own widely-extended homes will
have the best chance of seizing on new places, when they
spread into new countries. I n their new homes they
will be exposed to new conditions, and will frequently
undergo further modification and improvement ; andthus they will become still further victorious, and willproduce groups of modified descendants. On this principle of inheritance with modification, we can understand how it is that sections of genera, whole genera,

CHAP. XI.

SIXGLE CENTRES O F CREATION.

351

and even families are confined to the same areas, as isso commonly and notoriously the case.
I believe, as was remarked in the last chapter, in nolam of necessary development. As the variability ofeach species is an independent property, and will betaken advantage of by natural selection, only so far as
it profits the individual in its complex struggle forlife, so the degree of modification in different species
will be no uniform quantity. If, for instance, a numberof species, which stand in direct competition with eachother, migrate in a body into a new and afterwards
isolated country, they will be little liable to modification ; for neither migration nor isolation in themselves
can do anything. These principles come into play only
by bringing organisnis into new relations with each other,and in a lesser degree with the surrounding physical conditions. As we have seen in the last chapter that someforms have retained nearly the same character from anenormously remote geological period, so certain species
have migrated over vast spaces, and have not become
greatly modified.
On these views, it is obvious, that the several speciesof the same genus, though inhabiting the most distantquarters of the world, must originally have proceededfrom the same source, as they have descended from thesame progenitor. I n the case bf those species, whichhave undergone during whole geological periods but
little modification, there is not much difficulty in believing that they may have migrated from the same region ;for during the vast geographical and climatal changes
which will have supervened since ancient times, almost
any amount of migration is possible. Rut in many othercases, in which we have reason to believe that the speciesof a genus have been produced within comparatively
recent times, there is great difficulty on this head. It

352

GEOGRAPHICAL DISTRIBUTION.

CHAP. XI.

is also obvious that the individuals of the same species,
though now inhabiting distant and isolated regions, musthave proceeded from one spot, where their parents werefirst produced: for, as explained in the last chapter, it
is incredible that individuals identically the same should
ever have been produced through natural aelection fromparents specifically distinct.
We are thus brought to the question which has been
largely discussed by naturalists, namely, whether species
have been created a t one or more points of the earth's
surface. Undoubtedly there are very many cases ofextreme difficulty, in understanding how the same species could possibly have migrated from some one point
to the several distant and isolated points, where now
found. Nevertheless the simplicity of the view thateach species was first produced within a single region
captivates the mind. He who rejects it, rejects the
Vera cama of ordinary generation with subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a species is continuous ; and when a plantor animal inhabits two points so distant from eachother, or with an interval of such a nature, that the
space could not be easily passed over by migration, thefact is given as something remarkable and exceptional.
The capacity of migrating across the sea is more distinctly limited in terrestrial mammals, than perhaps inany other organic beings ; and, accordingly, we find no
inexplicable cases of the same mammal inhabiting distant points of the world. No geologist w i l l feel any
diaculty in such cases as Great Britain having been
formerly united to Europe, and consequently possessing
the same quadrupeds. But if the same species canbe produced at two separate points, why do we notfind a single mammal common to Europe and Australia or South America? The conditions of life areCmr. XI.

SINGLE CENTRES OF CREATION.

353

nearly the same, so that a multitude of European animals
and plants have become naturalised in America andAustralia ; and some of the aboriginal plants arc idcntia l l y the same at these distant points of the northernand southern hemispheres? The answer, as I believe,
is, that mammals have not been able to migrate, whereassome plants, from their varied means of dispersd, havemigrated across the vast and broken interspace. The
great and striking influence which barriers of every kind
have had on distribution, is intelligible only on the viewthat the great majority of specks have been producedon one side alone, and have not been able to migrate tothe other side. Some few families, many sub-fandies,very many genera, and a still greater number of sections of genera are confined to a sinde region ; and it
9
has been observed by several naturalists, that the mostnatural genera, or those genera in which the species aremost closely related to each other, are generally local,
or confined to one area. What a strange anomaly it
would be, if, when coming one stcp lower in the series,
to the individuals of the same species, a directly opposite rulc prevailed; and species were not local, but had
been produced in two or more distinct areas !
Herice it seems to me, as it has to many other naturalists, that the view of each species having been produced in one area alone, and having subsequently migrated from that area as far as its powers of migrationand subsistence under past and present conditions permitted, is the most probable. Undoubtedly many cases
occur, in which we cannot cxplain how the same species
could have passed from one point to the other. But
the geographical and climatal changes, which have certainly occurred within recent geological times, musthave interrupted or rendered discontinuous the formerly continuous range of many species. So that
we are reduced to consider wliethcr the exceptions to354

GEOGRAPHICAL DISTRIBUTION.

CHAP. XI.

continuity of range are so numerous and of so gravea nature, that we ought to give up the belief, rendered
probable by general considerations, that each species
has been produced within one area, and has migrated
thence as far as it could. It would be hopelessly tediousto discuss all the exceptional cases of the same species,
now living a t distant and separated points; nor do I
for a moment pretend that any explanation could beoffered of many such cases. But after some preliminary
remarks, I will discuss a few of the most striking classesof facts ; namely, the existence of the same species on
the summits of distant mountain-ranges, and a t distantpoints in the arctic and antarctic regions ; and secondly(in the following chapter), the wide distribution of freshwater productions ; and thirdly, the occuri-ence of thesame terrestrial species on islands and on the mainland,though separated by hundreds of miles of open sea. If
the existence of the same species a t distant and isolated
points of the earth's surface, can in many instances be
explained on the view of each species having migrated
from a single birthplace ; then, considering our ignorance with respect to former climatal and geographicalchanges and various occasional means of transport, thebelief that this has been the universal law, seems to me
incomparably the safest.
I n discussing this subject, we shall be enabled a t thesame time to consider a point equally important for us,
namely, whether the several distinct species of a genus,
which on my theory have all descended from a common
progenitor, can have migrated (undergoing modification
during some part of their migration) from the area
inhabited by their progenitor. If it can be shown tobe almost invariably the case, that a region, of whichmost of its inhabitants are closely related to, or belong
to the same genera with the species of a second region,

CHAP.XI.

SINGLE CENTRES OF CREATION.

355

has probably received a t some former period immigrants
from this other region, my theory will be strengthened ;for we can clearly understand, on the principle ofmodification, why the inhabitants of a region should be
related to those of another region, whence it has beenstocked. A volcanic island, for instance, upheaved andformed a t the distance of a few hundreds of miles from acontinent, would probably receive from it in the courseof time a few colonists, and their descendants, though
modified, would still be plainly related by inheritance tothe inhabitants of the continent. Cases of this natureare common, and are, as we shall hereafter more fullysee, inexplicable on the theory of independent creation.
This view of the relation of species in one region to
those in another, does not differ much (by substituting
the word variety for species) from that lately advanced
in an ingenious paper by Mr. Wallace, in which he concludes, that “ every species has come into existencecoincident both in space and time with a pre-existingclosely allied species.” And I now know from correspondence, that this coincidence he attributes to generation with modification.
The previous remarks on “single and multiple centresof creation” do not directly bear on another allied
question,-namely whether all the individuals of thesame species have descended from a single pair, or
single hermaphrodite, or whether, as some authors
suppose, from many individuals simultaneously created.
With those organic beings which never intercross (ifsuch exist), the species, on my theory, must have descended from a succession of improved varieties, which
will never have blended with other individuals or varieties, but will have supplanted each other ; so that, a t eachsuccessive stage of modification and improvement, all
the individuals of each variety will have descended from

356

GEOGRAPHICAL DISTRIBUTION.

CHAP.XI.

a single parent. But in the majority of cases, namely,
with all organisms which habitually unite for each birth,or which often intercross, I believe that during the slow
process of modification the individuals of the species will
have been kept nearly uniform by intercrossing ; so thatmany individuals will have gone on simultaneously
changing, and the whole amount of modification will nothave been due, at each stage, to descent from a single
parent. To illustrate what I mean : our English racehorses differ slightly from the horses of every otherbreed ; but they do not owe their difference and superiority to descent from any single pair, but to continued
care in select.ing and t.raining many individuals during
many generations.
Before discussing the three classes of facts, which
I have selected as presenting the greatest amount ofdifficulty on the theory of single centres of creation,”
I must say a few words on the means of dispersal.
((

Means of Dispersal.-Sir C. Lyell and other authors
have ably treated this subject. I can give here only
the briefest abstract of the more important facts.
Change of climate must have had a powerful influenceon migration : a region when its climate was different
may have been a high road for migration, but now be
impassable ; I shall, however, presently have to discuss
this branch of the subject in some detail. Changesof level in the land must also have been highly influential: a narrow isthmus now separates two marine
faunas ; submerge it, or let it formerly have been submerged, and the two faunas will now blend or may
formerly have blended: where the sea now extends,
land may at a former period have connected islands or
possibly even continents together, and thus have allowed
terrestrial productions to pass from one to the other.

CHAP. XI.

MEANS OF DISPERSAL.

357

No geologist will dispute that great mutations of level,
have occurred within the period of existing organisms.
Edward Forbes insisted that all the islands in the
Atlantic must recently have been connected with
Europe or Africa, and Europe likewise with America.
Other authors have thus hypothetically bridged overevery ocean, and have united almost every island tosome mainland. If indeed the arguments used by
Forbes are to be trusted, it must be admitted thatscarcely a single island exists which has not recently
been united to some continent. This view cuts theGordian knot of the dispersal of the same species to themost distant points, and removes many a difficulty : but
to the best of my judgment we are not authorized inadmitting such e n m o w geographical changes within
the period of existing species. It seems to me that wehave abundant evidence of great oscillations of level inour continents ; but not of such vast changes in theirposition and extension, as to have united them within
the recent period to each other and to the several intervening oceanic islands. I freely admit the former existence of many islands, now buried beneath the sea,
which may have served as halting places for plants
and for many animals during their migration. I n thecoral-producing oceans such sunken islands are now
marked, as I believe, by rings of coral or atolls standing
over them. Whenever it is fully admitted, as I believe
it will some day be, that each species has proceededfrom a single birthplace, and when in the course of time
we know something definite about the means of distribution, we shall be enabled to speculate with security
on the former extension of the land. But I do notbelieve that it will ever be proved that within the
recent period continents which are now quite separate,
have been continuously, or almost continuously, united

358

GEOGRAPHICAL DISTRIBUTION.

CHAP. XI.

with each other, and with the many existing oceanicislands. Several facts in distribution,-such as thegreat difference in the marine faunas on the opposite
sides of almost every continent,-the close relation of
the tertiary inhabitants of several lands and even seas totheir present inhabitants,-a certain degree of relation(as we shall hereafter see) between the distribution of
mammals and the depth of the sea,-theseand othersuch facts seem to me opposed to the admission of suchprodigious geographical revolutions within the recent
period, as are necessitated on the view advanced by
Forbes and admitted by his many followers. The
nature and relative proportions of the inhabitants ofoceanic islands likewise seem to me opposed to thebelief of their former continuity with continents. Nordoes their almost universally volcanic composition favour
the admission that they are the wrecks of sunken
continents ;-if they had originally existed as mountain-ranges on the land, some a t least of the islaiids
would have been formed, like other mountain-summits,of granite, metamorphic schists, old fossiliferous orother such rocks, instead of consisting of mere pilesof volcanic matter.
I must now say a few words on what are called accidental means, but which more properly might be calledoccasional means of distribution. I shall here confine
myself to plants. I n botanical works, this or that plantis stated to be ill adapted for wide dissemination ; but
for transport across the sea, the greater or less facilities
may be said to be almost wholly unknown. Until I
tried, with Mr. Berkeley’s aid, a few experiments, it
was not even known how far seeds could resist the injurious action of sea-water. To my surprise I found thatout of 87 kinds, 64 germinated after an immersion of
28 days, and a few survived an immersion of 137 days.

CHAP. XI.

MEANS OF DISPERSAL.

359

For convenience sake I chiefly tried small seeds,
without the capsule or fruit; and as all of these sankin a few days, they could not be floated across wide
spaces of the sea, whether or not they were injured by
the salt-water. Afterwards I tried some larger fruits,
capsules, &c., and some of these floated for a longtime. It is well known what a difference there is in thebuoyancy of green and seasoned timber; and it occurred
to me that floods might wash down plants or branches,
and that these might be dried on the banks, and thenby a fresh rise in the stream be washed into the sea.
Hence I was led to dry stems and branches of 94 plants
with ripe fruit, and to place them on sea water. The
majority sank quickly, but some which whilst green
floated for a very short time, when dried floated much
longer ; for instance, ripe hazel-nuts sank immediately,
but when dried, they floated for 90 days and afterwards
when planted they germinated ; an asparagus plantwith ripe berries floated for 23 days, when dried it
floated for 85 days, and the seeds afterwards germinated : the ripe seeds of Helosciadium sank in two days,
when dried they floated for above 90 days, and afterwards germinated Altogether out of the 94 driedplants, 18 floated for above 28 days, and some of the 1s
floated for a very much longer period. So that as #+seeds germinated after an inimersion of 28 days ; andas $9 plants with ripe fruit (but not all the same species
as in the foregoing experiment) floated, after being dried,
for above 28 days, as far as we may irifcr anything fromthese scanty facts, we may conclude that the seeds of&4c plants of any country might be floated by sea-currents during 28 days, and would retain their power ofgermination. I n Johnston’s Physical Atlas, the averagerate of the several Atlantic currents is 33 miles perdiem (some currents running a t the rate of 60 miles

360

GEOGRAPHICAL DISTRIBUTION.

CHAP.XI.

per diem); on this average, the seeds of +,& plants
belonging to one country might be floated across 924
miles of sea to another country ; and when stranded, if
blown to a favourable spot by an inland gale, they would
germinate.
Subsequently to my experiments, M. Martens tried
similar ones, but in a much better manner, for heplaced the seeds in a box in the actual sea, so that they
were alternately wet and exposed to the air like reallyfloating plants. He tried 98 seeds, mostly different
from mine ; but he chose many large fruits and likewise
seeds from plants which live near the sea; and thiswould have favoured the average length of their flotation and of their resistance to the injurious action of thesalt-water. On the other hand he did not previously
dry the plants or branches with the fruit ; and this, as
we have seen, would have caused some of them to havefloated much longer. The result was that # of hisseeds floated for 42 days, and were then capable of germination. But I do not doubt that plants exposed to
the waves would float for a less time than those protected from violent movement as in our experiments.
Therefore it would perhaps be safer to assume that theseeds of about #& plants of a flora, after having been
dried, could be floated across a space of sea 900 miles
in width, and would then germinate. The fact of thelarger fruits often floating longer than the small, is interesting; as plants with large seeds or fruit could
hardly be transported by any other means ; and Alph.
de Candolle has shown that such plants generally haverestricted ranges.
But seeds may be occasionally transported in anothermanner. Drift timber is thrown up on most islands,
even on those in the midst of the widest oceans; andthe natives of the coral-islands in the PacSc, procure

CHAP. XI.

MEANS OF DISPERSAL.

361

stones for their tools, solely from the roots of drifted
trees, these stones being a valuable royal tax. I findon examination, that when irregularly shaped stones
are embedded in the roots of trees, small parcels ofearth are very frequently enclosed in their interstices
and behind them,-so perfectly that not a particle
could be washed away in the longest transport: out of
one small portion of earth thus completely enclosed bywood in an oak about 50 years old, three dicotyledonous plants germinated : I am certain of the accuracyof this observation. Again, I can show that the carcasses of birds, when floating on the sea, sometimes
escape being immediately devoured; and seeds ofmany kinds in the crops of floating birds long retain
their vitality : peas and vetches, for instance, are killedby even a few days’ immersion in sea-water ; but sometaken out of the crop of a pigeon, which had floated on
artificial salt-water for 30 days, to my surprise nearly all
germinated.
Living birds can hardly fail to be highly effective
agents in the transportation of seeds. I could give
many facts showing how frequently birds of many kinds
are blown by gales to vast diatances across the ocean.
We may I think safely assume that under such circumstances their rate of flight would often be 35 miles anhour; and some authors have given a far higher estimate. I have never seen an instance of nutritious
seeds passing through the intestines of a bird; but
hard seeds of fruit will pass uninjured through eventhe digestive organs of a turkey. In the course of twomonths, I picked up in my garden 12 kinds of seeds,out of the excrement of small birds, and these seemed
perfect, and some of them, which I tried, germinated.
But the following fact is more important : the crops ofbirds do not secrete gastric juice, and do not in the362

GEOGRAPHICAL DISTRIBUTION.

CHAP. XI.

least injure, as I know by trial, the germination ofseeds ; now after a bird has found and devoured a large
supply of food, it is positively asserted that all the grains
do not pass into the gizzard for 12 or even 18 hours.
A bird in this interval might easily be blown to the distance of 500 miles, and hawks are known to look out
for tired birds, and the contents of their torn crops
might thus readily get scattered. Mr. Brent informs methat a friend of his had to give up flying carrier-pigeons
from France to England, as the hawks on the English
coast destroyed so many on their arrival. Some hawks
and owls bolt their prey whole, and after an intervalof from twelve to twenty hours, disgorge pellets, which,as I know from experiments made in the Zoological
Gardens, include seeds capable of germination. Someseeds of the oat, wheat, millet, canary, hemp, clover,and beet germinated after having been from twelve totwenty-one hours in the stomachs of different birds ofprey; and two seeds of beet grew after having beenthus retained for two days and fourteen hours. Freshwater fish, I find, eat seeds of many land and water
plants : fish are frequently devoured by birds, and thus
the seeds might be transported from place to place. I
forced many kinds of seeds into the stomachs of deadfish, and then gave their bodies to fishing-eagles, storks,
and pelicans; these birds after an interval of many
hours, either rejected the seeds in pellets or passed
them in their excrement; and several of these seeds
retained their power of germination. Certain seeds,
however, were always killed by this process.
Although the beaks and feet of birds are generally
quite clean, I can show that earth sometimes adheresto them : in one instance I removed twenty-two grainsof dry argillaceous earth from one foot of a partridge,
and in this earth there was a pebble quite as large asCHAP. XI.

MEANS OF DISPERSAL.

363

the seed of a vetch. Thus seeds might occasionally betransported to great distances ; for many facts could begiven showing that soil almost everywhere is charged
with seeds. Reflect for a moment on the millions ofquails which annually cross the Mediterranean ; and can
we doubt that the earth adhering to their feet would
sometimes include a few minute seeds? But I shallpresently have to recur to this subject.
As icebergs are known to be sometimes loaded withearth and stones, and have even carried brushwood,bones, and the nest of a land-bird, I can hardly doubtthat they must occasionally have transported seeds fromone part to another of the arctic and antarctic regions,
YS suggested by Lyell ; and during the Glacial period
from one part of the now temperate regions to another.
In the Azores, from the large number of the speciesof plants common to Europe, in comparison with theplants of other oceanic islands nearer to the mainland,and (as remarked by Mr. H. C. Watson) from the somewhat northern character of the flora in comparison withthe latitude, I suspected that these islands had been
partly stocked by ice-borne seeds, during the Glacial
epoch. At my request Sir C. Lyell wrote to M. Hartungto inquire whether he had observed erratic boulders
on these islands, and he answered that he had foundlarge fragments of granite and other rocks, which do
not occur in the archipelago. Hence we may safely
infer that icebergs formerly landed their rocky burthenson the shores of these mid-ocean islands, and it is atleast possible that they may have brought thither theseeds of northern plants.
Considering that the several above means of transport, and that several other means, which without
doubt remain to be discovered, have been in action
year after year, for centuries and tens of thousands of364

GEOURAPHICAL DISTRIBUTION,

CHAP. XI.

years, it would I think be a marvellous fact if many
plants had not thus become widely transported. Thesemeans of transport are sometimes called accidental, but
this is not strictly correct: the currents of the sea are
not accidental, nor is the direction of prevalent galesof wind. It should be observed that scarcely anymeans of transport would carry seeds for very great
distances ; for seeds do not retain their -vitality when
exposed for a great length of time to the action of seawater; nor could they be long carried in the crops orintestines of birds. These means, however, would sufficefor occasional transport across tracts of sea some hundred miles in breadth, or from island to island, or froma continent to a neighbouring island, but not from one
distant continent to another. The floras of distantcontinents would not by such means become mingled
in any great degree; but would remain as distinct
as we now see them to be. The currents, from theircourse, would never bring seeds from North America
to Britain, though they might and do bring seeds
from the West Indies to our western shores, where,
if not killed by so long an immersion in salt-water,
they could not endure our climate. Almost every
year, one or two land-birds are blown across thewhole Atlantic Ocean, from North America to thewestern shores of Ireland and England; but seeds
could be transported by these wanderers only by one
means, namely, in dirt sticking to their feet, whichis in itself a rare accident. Even in this case, howsmall would the chance be of a seed falling on favourable soil, and coming to maturity! But it would bea great error to argue that because a well-stocked
island, like Great Britain, has not, as far as is known(and it would be very diEicult to prove this), received
within the last few centuries, through occasional means

CHAP. XI.

365

DURING THE GLACIAL PERIOD.

of transport, immigrants from Europe or any othercontinent, that a poorly-stocked island, though standing
more remote from the mainland, would not receive
colonists by similar means. I do not doubt that out oftwenty seeds or animals transported to an island, evenif far less well-stocked than Britain, scarcely more than
one would be so well dtted to its new home, as tobecome naturalised. But this, as it seems to me, isno valid argument against what would be effected byoccasional means of transport, during the long lapse ofgeological time, whilst an island was being upheaved
and formed, and before it had become fully stockedwith inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every
seed, which chanced to arrive, would be sure to germinate and survive.
Dispersal during the Glacial period.-The identity ofmany plants and animals, on mountain-summits, separated from each other by hundreds of miles of lowlands, where the Alpine species could not possibly exist,
is one of the most striking cases known of the same
species living a t distant points, without the apparent
possibility of their having migrated from one to theother. It is indeed a remarkable fact to see so manyof the same plants living on the snowy regions of theAlps or Pyrenees, and in the extreme northern partsof Europe; but it is far more remarkable, that theplants on the White Mountains, in the United States ofAmerica, are all the same with those of Labrado?, andnearly all the same, as we hear from Asa Gray, with /thoseon the loftiest mountains of Europe. Even as lon ago
as 1747, such facts led Gmelin to conclude that thesame species must have been independently created a t
several distinct points; and we might have remained

i:

366

GEOGRAPHICAL DISTRIBUTIOX,CIiAP. XI.

in this same belief, had not Agassiz and others called
vivid attention to the Glacial period, which, as we shallimmediately see, affords a simple explanation of thesefacts. We have evidence of almost every conceivablekind, organic and inorganic, that within a very recent
geological period, central Europe and North America
suffered under an Arctic climate. The ruins of a houseburnt by fire do not tell their tale more plainly, than
do the mountains of Scotland and U7ales, with theirscored flanks, polished surfaces, and perched boulders,of the icy streams with which their valleys were lately
filled. So greatly has the climate of Europe changed,that in Northern Italy, gigantic moraines, left by old
glaciers, are now clothed by the vine and maize. Throughout a large part of the United States, erratic boulders,
and rocks scored by drifted icebergs and coast-ice, plainly
reveal a former cold period.
The former influelice of the glacial climate on the
distribution of the inhabitants of Europe, as explainedwith remarkable clearness by Edmard Forbes, is substantially as follows. But we shall follow the changes
more readily, by supposing a new glacial period to comeslowly on, and then pass away, as formerly occurred. As
the cold came on, and as each more southern zone became
fitted for arctic beings and ill-fitted for their former
more temperate inhabitants, the latter wonld be supplanted and arctic productions would take their places.
The inhabitants of the more temperate regions woulda t the same time travel southward, unless they were
stopped by barriers, in which case they would perish.
The mountaiiis would become covered with snow andice, and their former Alpine inhabitants would descend
to the plains. By the time that the cold had reached
its maximum, we should have a uniform arctic faunaand flora, covering the central parts of Europe, as far

CHAP. XI.

DURING THE GLACIAL PERIOD.

367

south as the Alps and Pyrenees, and even stretching
into Spain. The now temperate regions of the UnitedStates would likewise be covered by arctic plants andanimals, and these would be nearly the same with thoseof Europe ; for the present circumpolar inhabitants,
which we suppose to have everywhere travelled southward, are remarkably uniform round the world. We
may suppose that the Glacial period came on a little
earlier or later in North America than in Europe, so
will the southern migration there have been a little
earlier or later ; but this will make no difference in the
final result.
As the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by
the productions of the more temperate regions. And as
the snow melted from the bases of the mountains, thearctic forms would seize on the cleared and thawed
ground, always ascending higher and higher, as thewarmth increased, whilst their brethren were pursuingtheir northern journey. Hence, when the warmth had
fully returned, the same arctic species, which had lately
lived in a body together on the lowlands of the Old and
New Worlds, would be left isolated on distant mountainsummits (having been exterminated on all lesser heights)
and in the arctic regions of both hemispheres.
Thus we can understand the identity of many plants
at points so immensely remote as on the mountainsof the United States and of Europe. We can thus
also understand the fact that the Alpine plants ofeach mountain-range are more especially related tothe arctic forms living due north or nearly due northof them: for the migration as the cold came on, and there-migration on the returning warmth, will generally
have been due south and north. The Alpine plants, forexample, of Scotland, as remarked by Nr. H. C. Watson,

368

GEOGRAPHICAL DISTRIBUTIOX,

CHAP. XI.

and those of the Pyrenees, as remarked by Ramond, aremore especially allied to the plants of northern Scandinavia ; those of the United States to Labrador ; thoseof the mountains of Siberia to the arctic regions of thatcountry. These views, grounded as they are on theperfectly well-ascertained occurrence of a former Glacial
period, seein to me to explain in so satisfactory amanner the present distribution of the Alpine andarctic productions of Europe and America, that when inother regions we find the same species on distant mountain-summits, w7e may almost conclude without otherevidence, that a colder climate permitted their forniermigration across the low intervening tracts, since become too warm for their existence.
If the climate, since the Glacial period, has ever beenin any degree warmer than a t present (as some geologists in the United States believe to have been thecase, chiefly from the distribution of the fossil Gnathodon), then the arctic and temperate productions will a t
a very late period have marched a little further north,and subsequently have retreated to their present homes ;but I have met with no satisfactory evidence with respect
to this intercalated slightly warmer period, since the
Glacial period.
The arctic forms, during their long southern migration and re-migration northward, will have been exposed
to nearly the same climate, and, as is especially to be
noticed, they will have kept in a body together; consequently their. mutual relations will not have beenmuch disturbed, and, in accordance with the principles
inculcated in this volume, they will not have been liable
to much modification. But with our Alpine productions,left isolated from the moment of the returning warmth,
first at the bases and ultimately on the suinmits ofthe mountains, the case will have been somewhat dif-

CHAP. XI.

DURING THE GLACIAL PERIOD.

369

ferent ; for it is not likely that all the same arctic species will have been left on mountain ranges distant fromeach other, and have survived there ever since ; they
will, also, in all probability have become mingled withancient Alpine species, which must have existed on
the mountains before the commencement of the Glacial
epoch, and which during its coldest period will hayebeen temporarily driven down to the plains ; they will,
also, have been exposed to somewhat different climatalinfluences. Their mutual relations will thus have beenin some degree disturbed ; consequently they will havebeen liable to modification ; and this we find has been
the case ; for if we compare the present Alpine plants
and animals of the several great European mountainranges, though very many of the species are identicallythe same, some present varieties, some are ranked as
doubtful forms, and some few are distinct yet closely
allied or representative species.
I n illustrating what, as I believe, actually took place
during the Glacial period, I assumed that a t its commencement the arctic productions were as uniform
round the polar regions as they are at the present day.
But the foregoing remarks on distribution apply not
only to strictly arctic forms, but also to many sub-arctic
and to some few northern temperate forms, for some ofthese are the same on the lower mountains and on theplains of North America and Europe ; and it may bereasonably asked how I account for the necessary degree of uniformity of the sub-arctic and northern temperate forms round the world, at the commencement of
the Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and
New Worlds are separated from each other by the
Atlantic Ocean and by the extreme northern part of
the Pacific. During the Glacial period, when the in-

370

GEOGRAPHICAL DISTRIBUTION,

CHAP. XI.

habitants of the Old and New Worlds lived further
southwards than a t present, they must have been still
more completely separated by wider spaces of ocean.
I believe the above difficulty may be surmounted bylooking to still earlier changes of climate of an opposite
nature. We have good reason to believe that during
the newer Pliocene period, before the Glacial epoch,
and whilst the majority of the inhabitants of the world
were specifically the same as now, the climate was
warmer than a t the present day. Hence we may suppose that the organisms now living under the climateof latitude 60°, during the Pliocene period lived further
north under the Polar Circle, in latitude 66"-87' ; andthat the strictly arctic productions then lived on thebroken land still nearer to the pole. Now if we look a t
a globe, we shall see that under the Polar Circle there
is almost continuous land from western Europe, throughSiberia, to eastern America. And to this continuity of
the circumpolar land, and to the consequent freedom
for intermigration under a more favourable climate, I
attribute the necessary amount of uniformity in the
sub-arctic and northern temperate productions of the
Old and New Worlds, a t a period anterior to the Glacial
epoch.
Believing, from reasons before alluded to, that ourcontinents have long remained in nearly the same relative position, though subjected to large, but partial
oscillations of level, I am strongly inclined to extend
the above view, and to infer that during some earlier
and still warmer period, such as the older Pliocene
period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land ; andthat these plants and animals, both in the Old and
New T;STorlds, began slowly to migrate southwards as
the climate became less warm, long before the com-

CHAP.XI.

DURING T H E GLACIAL PERIOD.

371

mencement of the Glacial period. W e now see, a s 1
believe, their descendants, mostly in a modified condition, in the central parts of Europe and the UnitedStates. On this view we can understand the relationship, with very little identity, between the productionsof North America and Europe,-a relationship which ismost remarkable, considering the distance of the twoareas, and their separation by the Atlantic Ocean. We
can further understand the singular fact remarked on
by several observers, that the productions of Europe
and America during the later tertiary stages were mor2
closely related to each other than they are at the pTesent time ; for during these warmer periods the northernparts of the Old and New Worlds mill have been almost
continuously united by land, serving as a bridge, sincerendered impassable by cold, for the inter-migration of
their inhabitants.
During the slowly decreasing warmth of the Pliocene
period, as soon as the species in common, which inhabitedthe New and Old Worlds, migrated south of the l’olar
Circle, they must have been completely cut off from eachother. This separation, as far as the more temperate productions are concerned, took place long ages ago. And
as the plants and animals migrated southward, they will
have become mingled in the one great region with thenative American productions, and have had to competewith them; and in the other great region, with thoseof the Old World. Consequently we have here everything favowable for much modification,-for far more
modification than with the Alpine productions, left
isolated, within a much more recent period, on theseveral mountain-ranges and on the arctic lands of thetwo Worlds. Hence it has come, that when we compare
the now living productions of the temperate regions of
the New and Old Worlds, we find very few identical

372

GEOGRAPHICAL DISTRIBUTION,

CHAP.XI.

species (though Asa Gray has lately shown that more
plants are identical than was formerly supposed), but
we find in every great class many forms, which somenaturalists rank as geographical races, and others as distinct species ; and a host of closely allied or representative forms which are ranked by all naturalists as
specifically distinct.
As on t,he land, so in the waters of the sea, a slow
southern migration of a marine fauna, which during
the Pliocene or even a somewhat earlier period, wasnearly uniform along the continuous shores of the Polar
Circle, will account, on the theory of modification, formany closely allied forms now living in areas completely
sundered. Thus, I think, we can understand the presence of many existing and tertiary representative forms
on the eastern and western shores of temperate North
America; and the still more striking case of manyclosely allied crustaceans (as described in Dana’s admirable work), of some fish and other marine animals, inthe Mediterranean and in the seas of Japan,-areas
now separated by a continent and by nearly a hemisphere of equatorial ocean.
These cases of relationship, without identity, of the
inhabitants of seas now disjoined, and likewise of thepast and present inhabitants of the temperate lands ofNorth America and Europe, are inexplicable on thetheory of creation. We cannot say that they havebeen created alike, in correspondence with the nearly
similar physical conditions of the areas ; for if we compare, for instance, certain pai-ts of South America withthe souther2 continents of the Old World, we see
countries closely corresponding in all their physicalconditions, but with their inhabitants utterly dissimilar.
But we must return to our more immediate subject,
the Glacial period. I am convinced that Forbes’s view

CHAP.XI.

DURIBG THE GLACIAL PEBIOD.

373

may be largely extended. I n Europe we have theplainest evidence of the cold period, from tlie western
shores of Britain to the Oural range, and southward to
the Pyrenees. We may infer, from the frozen inamnials
and nature of the niountain vegetation, that Siberia was
simiIarly affected. Along the Himalaya, a t points 900
miles apart, glaciers have left the marks of their former
low descent; and in Sikkim, Dr. Hooker saw maize
growing on gigantic ancicizt moraines. South of theequator, we have some direct evidence of former glacial
actioiz in New Zealand ; and the same plants, found on
widely separated mountains in this island, tell the same
story. If one account which has been published can betrusted, we have direct evidence of glacial action in thesouth-eastern corner of Australia.
Looking to America ; in the northern half, ice-borne
fragments of rock have been observed on the easternside as far south as lat. 3G0-370, and on the shores of
the Pacific, where the climate is now so different, as
far south as lat. 46' ; erratic boulders have, also, been
noticed on the Rocky Mountains. I n the Cordillera
of Equatorial South America, glaciers once extended
far below their present level. I n central Chile I was
astonished at the structure of a vast mound of detritus,
about 800 feet in height, crossing a valley of theAndes; and this I now feel convinced was a gigantic
moraine, left far below any existing glacier. Further
south on both sides of the continent, from lat. 41' tothe southernmost extremity, we have the clearest
evidence of former glacial action, in Iiuge boulders
transported far from their parent source.
We do not know that the Glacial epoch was strictly
simultaneous a t these several far distant points on opposite sides of the world. But we have good evidence inalmost every case, that the epoch was included within

374

GEOGRAPHICAL DISTRIBUTION,

CHAP. XI.

the latest geological period. We have, also, excellent
evidence, that it endured for an enormous time, as
measured by years, a t each point. The cold may havecome on, or have ceased, earlier a t one point of the
globe than at another, but seeing that it endured forlong a t each, and that it was contemporaneous in ageological sense, it seems to me probable that it was,during a part a t least of the period, actually simultaneous throughout the world. Without some distinct
evidence to the contrary, we may at least admit asprobable that the glacial action was simultaneous on
the eastern and western sides of North America, in the
Cordillera under the equator and under the warmer
temperate zones, and on both sides of the southern
extremity of the continent. If this be admitted, it isdiEcult to avoid believing that the temperature of
the whole world was at this period simultaneously
cooler. But it would suffice for my purpose, if thetemperature was at the same time lower along certain
broad belts of longitude.
On this view of the whole world, or at least of broad
longitudinal belts, having been simultaneously colder
from pole to pole, milch light can be thrown on thepresent distribution of identical and allied species. In
America, Dr. Hooker has shown that between forty andfifty of the flowering plants of Tierra del Fuego, forming
no inconsiderable part of its scanty flora, are common toEurope, enormously remote as these two points are ; andthere are many closely allied species. On the lofty
mountains of equatorial America a host of peculiar
species belonging to European genera occur. On thehighest mountains of Brazil, some few European genem
were found by Gardner, which do not exist in the wide
intervening hot countries. So on the Silla of Caraccas
the illustrious Humboldt long ago found species belong-

CHAP. XI.

DURING THE GLACIAL PERIOD.

375

ing to genera characteristic of the Cordillera. On themountains of Abyssinia, several European forms andsome few representatives of the peculiar flora of theCape of Good Hope occur. At the Cape of Good Hopea very few European species, believed not to have been
introduced by man, and on the mountains, some few
representative European forms are found, which havenot been discovered in the intertropical parts of Africa.
On the Himalaya, and on the isolated mountain-rangesof the peninsula of India, on the heights of Ceylon, andon the volcanic cones of Java, many plants occur, either
identically the same or representing each other, and a t
the same time representing plants of Europe, not foundin the intervening hot lowlands. A list of the genera
collected on the loftier peaks of Java raises a pictureof a collection made on a hill in Europe! Still morestriking is the fact that southern Australian forms areclearly represented by plants growing 011 the surnmitsof the mountains of Borneo. Some of these Australian
forms, as I hear from Dr. Hooker, extend along theheights of the peninsula of Malacca, and are thinly
scattered, on the one hand over India and on the otheras far north as Japan.
On the southern mountains of Australia, Dr. F.
Miiller has discovered several European species ; otherspecies, not introduced by man, occur on the lowlands ;and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but
not in the intermediate torrid regions. I n the admirable ‘Introduction to the Flora of New Zealand,’ by
Dr. Hooker, analogous and striking facts are given in
regard to the plants of that large island. Hence we seethat throughout the world, the plants growing on the
more lofty mountains, and on the temperate lowlandsof the northern and southern hemispheres, are sometimes

376

GEOGRAPHICAL DISTRIBUTION,

CHAP. XI.

identically the same ; but they are much oftener specifically distinct, though related to each other in a mostremarkable manner.
This brief abstract appIies t o plants alone: somestrictly analogous facts could be given on the distribution of terrestrial animals. I n marine productions,similar cases occur; as an example, I may quote a
remark by the highest authority, Prof. Dana, that ‘‘ itis certainly a wonderful fact that New Zealand shouldhave a closer resemblance in its crustacea to GreatBritain, its antipode, than to any other part of theworld.” Sir J. Richardson, also, speaks of the reappearance on the shores of New Zealand, Tasmania,&c., of northern forms of fish. Dr. Hooker informs
me that twenty-five species of Alga? are common to
New Zealand and to Europe, but have not been foundin the intermediate tropical seas.
It should be observed that the northern species andforms found in the southern parts of the southern hemisphere, and on the mountain-ranges of the intertropicalregions, are not arctic, but beIong to the northern femperate zones. As Mr. H. C. Watson has recently remarked, (‘I n receding from polar towards equatorial
latitudes, the Alpine or mountain floras really becomeless and less arctic.” Many of the forms living on themountains of the warmer regions of the earth and in
the southern hemisphere are of doubtful value, being
ranked by some naturalists as specifically distinct, byothers as varieties ; but some are certainly identical,
and many, though closely related to northern forms,
must be ranked as distinct species.
Now let us see what light can be thrown on the foregoing facts, on the belief, supported as it is by a large
body of geological evidence, that the whole world, or alarge part of it, was during the Glacial period simulta-

CHAP. XI.

DURING THE GLACIAL PERIOD,

377

neously much colder than at present. The Glacial
period, as measured by years, must have been very
long; and when we remember over what vast spacessome naturalised plants and animals have spread withina few centuries, this period will have been ample forany amount of migration. As the cold came slowly on,
all the tropical plants and other procluctions will haveretreated from both sides towards the equator, followedin the rear by the temperate productions, and these by
the arctic; but with the latter we are not now concerned. The tropical plants probably suffered muchextinction; how much no one can say; perhaps formerly the tropics supported as many species as we seeat the present day crowded together a t the Cape ofGood Hope, and in parts of temperate Australia. As
we know that many tropical plants and animals canwithstand a considerable amount of cold, many might
have escaped extermination during a moderate fall of
temperature, more especially by escaping into thewarmest spots. But the great fact to bear in mind is,that all tropical productions will have suffered to a certain extent. On the other hand, the temperate productions, after migrating nearer to the equator, though
they will have been placed under somewhat new conditions, will have suffered less. And it is certain thatmany temperate plants, if protected from the inroadsof competitors, can withstand a much warmer climate
than their own. Hence, it seems to me possible,
bearing in mind that the tropical productions werein a suffering state and could not have presented afirm front against intruders, that a certain number ofthe more vigorous and dominant temperate forms might
have penetrated the native ranks and have reached oreven crossed the equator. The invasion would, of course,
have been greatly favoured by high land, and perha1)s378

GEOGRAPHICAL DISTRIBUTION,

CHAP. XI.

by a dry climate ; for Dr. Falconer informs me that itis the damp with the heat of the tropics which is so
destructive to perennial plants from a temperate climate. On the other hand, the most humid and hottest
districts will have afforded an asylum to the tropicalnatives. The mountain-ranges north-west of the Himalaya, and the long line of the Cordillera, seem to haveafforded two great lines of invasion : and it is a striking
fact, lately communicated to me by Dr. Hooker, that all
the flowering plants, about forty-six in number, common
to Tierra del Fuego and to Europe still exist in North
America, which must have lain on the line of march.
But I do not doubt that some temperate productions
entered and crossed even the lowlands of the tropics a t
the period when the cold was most intense,-whenarctic forms had migrated some twenty-five degrees
of latitude from their native country and covered theland a t the foot of the Pyrenees. At this period of extreme cold, I believe that the climate under the equatora t the level of the sea was about the same with that now
felt there a t the height of six or seven thousand feet.
During this the coldest period, I suppose that large
spaces of the tropical lowlands were clothed with amingled tropical and temperate vegetation, like thatnow growing with strange luxuriance a t the base of theHimalaya, as graphically described by Hooker.
Thus, as I believe, a considerable number of plants, a
few terrestrial animals, and some marine productions,migrated during the Glacial period from the northernand southern temperate zones into the intertropical regions, and some even crossed the equator. As the warmth
returned, these temperate forms would naturally ascend
the higher mountains, being exterminated on the lowlands ; those which had not reached the equator, would
re-migrate northward or southward towards their former

CHAP. XI.

DURING THE GLACIAL PERIOD.

379

homes; but the forms, chiefly northern, which hadcrossed the equator, would travel still further from theirhomes into the more temperate latitudes of the opposite
hemisphere. Although we have reason to believe fromgeological evidence that the whole body of arctic shells
underwent scarcely any modification during their longsouthern migration and re-migration northward, the case
may have been wholly different with those intruding
forms which settled themselves on the intertropicalmountains, and in the southern hemisphere. These
being surrounded by strangers will have had to competewith many new forms of life; and it is probable that
selected modifications in their structure, habits, and constitutions will have profited them. Thus many of thesewanderers, though still plainly related by inheritance totheir brethren of the northern or southern hemispheres,
now exist in their new homes as well-marked varieties
or as distinct species.
It is a remarkable fact, strongly insisted on by
Hooker in regard to America, and by Alph. de Candolle
in regard to Australia, that many more identical plants
and allied forms have apparently migrated from thenorth to the south, than in a reversed direction. We
see, however, a few southern vegetable forms on themountains of Borneo and Abyssinia. I suspect that
this preponderant migration from north to south is dueto the greater extent of land in the north, and to thenorthern forms having existed in their own homes ingreater numbers, and having consequently been advanced through natural selection and competition to ahigher stage of perfection or dominating power, than thesouthern forms. And thus, when they became commingled during the Glacial period, the northern forms
were enabled to beat the less powerful southern forms.
Just in the same manner as we see a t the present day,

380

GEOGRAPHICAL DISTRIBUTION,

CHAP. XI.

that very many European productions cover the groundin La Plata, and in a lesser degree in Australia, andhave to a certain extent beaten the natives ; whereas
extremely few southern forms have become naturalisedin any part of Europe, though hides, wool, and otherobjects likely to carry seeds have been largely imported into Europe during the last two or three centuries from La Plata, and during the last thirty or forty
years from Australia. Something of the same kindmust have occurred on the intertropical mountains : no
doubt before the Glacial period they were stocked withendemic Alpine forms ; but these have almost everywhere largely yielded to the more dominant forms,
generated in the larger areas and more efficient workshops of the north. I n many islands the native productions are nearly equalled or even outnumbered by
the naturalised ; and if the natives have not been actually exterminated, their numbers have been greatly
reduced, and this is the first stage towards extinction.
A mountain is an island on the land; and the intertropical mountains before the Glacial period must havebeen completely isolated ; and I believe that the productions of these islands on the land yielded to those
produced within the larger areas of the north, just in
the same way as the productions of real islands haveeverywhere lately yielded to continental forms, naturalised by man’s agency.
I am far from supposing that all difficulties are removed on the view here given in regard to the range
and affinities of the allied species which live in thenorthern and southern temperate zones and on themountains of the intertropical regions. Very manydifficulties remain t o be solved. I do not pretend toindicate the exact lines and means of migration, or thereason why certain species and not others have migrated;

CHAP. XI.

DURING THE GLACIAL PERIOD.

381

why certain species have been modified and have givenrise to new groups of forms, and others have remained
unaltered. We cannot hope to explain such facts,
until we can say why one species and not anot,her becomes naturalised by man’s agency in a foreign land ;why one ranges twice or thrice as far, and is twice or
thrice as common, as another species within their own
homes.
I have said that many difficulties remain to be solved :some of the most remarkable are stated with admirable
clearness by Dr. Hooker in his botanical works on theantarctic regions. These cannot be here discussed. I
will only say that as far as regards the occurrence ofidentical species at points so enormously remote as
Kerguelen Land, New Zealand, and Fuegia, I believethat towards the close of the Glacial period, icebergs,
as suggested by Lyell, have been largely concerned intheir dispersal. But the existence of several quitedistinct species, belonging to genera exclusively confinedto the south, at these and other distant points of thesouthern hemisphere, is, on my theory of descent withmodification, a far more remarkable case of difficulty.
For some of these species are so distinct, that we cannot
suppose that there has been time since the commencement of the Glacial period for their migration, andfor their subsequent modification to the necessary
degree. The facts seem t o me to indicate that peculiar and very distinct species have migrated in radi
ating lines from some common centre ; and I am inclined to look in the southern, as in the northern hemisphere, to a former and waimer period, before the commencement of the Glacial period, when the antarctic
lands, now covered with ice, supported a highly peculiar
and isolated flora. I suspect that before this flora wasexterminated by the Glacial epoch, a few forms were

382

GEOGRAPHICAL DISTRIBUTION.

CHAP. XI.

widely dispersed to various points of the southernhemisphere by occasional means of transport, and by
the aid, as halting-places, of existing and now sunken
islands, and perhaps at the commencement of the
Glacial period, by icebergs. By these means, as I believe, the southern shores of America, Australia, New
Zealand have become slightly tinted by the same peculiar forms of vegetable life.
Sir C. Lyell in a striking passage has speculated, in
language almost identical with mine, on the effects ofgreat alternations of climate on geographical distribution. I believe that the world has recently felt oneof his great cycles of change; and that on this view,combined with modification through natural selection,a multitude of facts in the present distribution bothof the same and of allied forms of life can be explained. The living waters may be said to have flowed
during one short period from the north and from thesouth, and to have crossed at the equator; but tohave flowed with greater force from the north so as tohave freely inundated the south. As the tide leaves
its drift in horizontal lines, though rising higher on
the shores where the tide rises highest, so have theliving waters left their living drift on our mountainsummits, in a line gently rising from the arctic ~ Q W land9 to a great height under the equator. The various
beings thus left stranded may be compared with savage
races of man, driven up and surviving in the mountainfastnesses of almost every land, which serve as a record,
full of interest to us, of the former inhabitants of thesurrounding lowlands.

C H A P . XII.

FRESH-WATER PRODUCTIONS.

383

C H A P T E R XII.
GEOQRAPHICAL~IsTRIBuTroh’-co?~ti~e~.

-

Distribution of fresh-water productions
On the inhabitants ofoceanic islands - Absence of Batrachians and of terrestrial Mammals - On the relation of the inhabitants of islands to those of
the nearest mainland - On colonisation from the nearest sourcewith subsequent modification Snmmary of the last and present chapters.

-

As lakes and river-systems are separated from eachother by barriers of land, it might have been thoughtthat fresh-water productions would not have ranged
widely within the same country, and as the sea is apparently a still more impassable barrier, that they
never would have extended to distant countries. But
the case is exactly the reverse. Not only have many
fresh-water species, belonging to quite different classes,
an enormous range, but allied species prevail in aremarkable manner throughout the world. I well remember, when first collecting in the fresh waters ofBrazil, feeling much surprise a t the similarity of the
fresh-water insects, shells, &c., and a t the dissimilarityof the surrounding terrestrial beings, compared withthose of Britain.
But this power in fresh-water productions of ranging
widely, though so unexpected, can, I think, in mostcases be explained by their having become fitted, ina manner highly useful to them, for short and frequentmigrations from pond to pond, or from stream tostream; and liability to wide dispersal would follow
from this capacity as an almost necessary consequence.
We can here consider only a few cases. In regard to

384

GEOGRAPHICAL DISTRIBUTION.

CHAP. XII.

fish, I believe that the same species never occur inthe fresh waters of distant continents. But on thesame continent the species often range widely andalmost capriciously ; for two river-systems will havesome fish in common and some different. A few facts
seem to favour the possibility of their occasional transport by accidental means ; like that of the live fish not
rarely dropped by whirlwinds in India, and the vitalityof their ova when removed from the water. But I am
inclined to attribute the dispersal of fresh-water fish
mainly to slight changes within the recent period in
the level of the land, having caused rivers to flow into
each other. Instances, also, could be given of thishaving occurred during floods, without any change oflevel. We have evidence in the loess of the Rhine ofconsiderable changes of level in the land within a very
recent geological period, and when the surface waspeopled by existing land and fresh-water shells. The
wide difference of the fish on opposite sides of continuous mountain-ranges, which from an early period
must have parted river-systems and completely prevented their inosculation, seems t o lead to this sameconclusion. With respect to allied fresh-water fishoccurring at very distant points of the world, no doubt
there are many cases which cannot at present be explained: but some fresh-water fish belong to veryancient forms, and in such cases there will have been
ample time for great geographical changes, and consequently time and means for much migration. I n
the second place, salt-water fish can with care be slowly
accustomed to live in fresh water; and, according to
Valenciennes, there is hardly a single group of fishes
confined exclusively to fresh water, so that we may
imagine that a marine member of a fresh-water group
might travel far along the shores of the sea, and subse-

CHAP.XII.

FRESH-WATER PRODUCTIONS.

385

quently become modified and adapted to the fresh
waters of a distant land.
Some species of fresh-water shells have a very wide
range, and allied species, which, on my theory, are descended from a common parent and must have proceededfrom a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their
ova are not likely t o be transported by birds, and they
are immediately killed by sea water, as are the adults.
I could not even understand how some naturalised
species have rapidly spread throughout the same
country. But two facts, which I have observed-andno doubt many others remain to be observed-throw
some light on this subject. When a duck suddenly
emerges from a pond covered with duck-weed, I have
twice seen these little plants adhering t o its back; andit has happened to me, in removing a little duckweed from one aquarium to another, that I have quite
unintentionally stocked the one with fresh-water shells
from the other. But another agency is perhaps more
effectual: I suspended a duck's feet, which might
represent those of a bird sleeping in a natural pond,in an aquarium, where many ova of fresh-water shells
were hatching; and I found that numbers of the extremely minute and just hatched shells crawled on thefeet, and clung to them so firmly that when taken outof the water they could not be jarred off, though a t
a somewhat more advanced age they would voluntarily
drop off. These just hatched molluscs, though aquaticin their nature, survived on the duck's feet, in damp
air, from twelve to twenty hours ; and in this length oftime a duck or heron might fly at least six or seven
hundred miles, and would be sure to alight on a poolor rivulet, if blown across sea to an oceanic island or
to any other distant point. Sir Charles Lye11 also

386

GEOGRAPHICAL DISTRIBUTION.

CHAP. XII.

informs me that a Dyticus has been caught with anAncylus (a fresh-water shell like a limpet) firmly adhering to it; and a water-beetle of the same family, aColymbetes, once flew on board the 'Beagle,' when
forty-five miles distant from the nearest land: howmuch farther it might have flown with a favouring galeno one can tell.
With respect to plants, it has long been known what
enormous ranges many fresh-water and even marshspecies have, both over continents and to the mostremote oceanic islands. This is strikingly shown, as
remarked by Alph. de Candolle, in large groups ofterrestrial plants, which have only a very few aquatic
members; for these latter seem immediately to acquire,as if in consequence, a very wide range. I think favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally, though rarely, adheres in some quantity to the feet and beaks of birds.
Wading birds, which frequent the muddy edges ofponds, if suddenly flushed, would be the most likely tohave muddy feet. Birds of this order I can show are
the greatest wanderers, and are occasionally found on
the most remote and barren islands in the open ocean ;they would not be likely to alight on the surface of thesea, so that the dirt would not be washed off their feet;when making land, they would be sure to fly to theirnatural fresh-water haunts. I do not believe thatbotanists are aware how charged the mud of ponds iswith seeds : I have tried several little experiments, butwill here give only the most striking case: I took in
February three table-spoonfuls of mud from three different points, beneath water, on the edge of a little
pond; this mud when dry weighed only 6$ ounces; I
kept it covered up in my study for six months, pullingup and counting each plant as it grew ; the plants wereCHAP.XII.

FRESH-WATER PRODUCTIONS.

387

of many kinds, and were altogether 537 in number;and yet the viscid mud was all contained in a breakfast
cup! Considering these facts, I think it would be an
inexplicable circumstance if water-birds did not transport the seeds of fresh-water plants to vast distances,and if consequently the range of these plants was notvery great. The same agency may have come intoplay with the eggs of some of the smaller fresh-water
animals.
Other and unknown agencies probably have also
played a part. I have stated that fresh-water fish eatsome kinds of seeds, though they reject many otherkinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow waterlily and Potamogeton. Herons and other birds, century
after century, have gone on daily devouring fish ; they
then take flight and go to other waters, or are blown
across the sea ; and we have seen that seeds retain theirpower of germination, when rejected in pellets or in
excrement, many hours afterwards. When I saw thegreat size of the seeds of that fine water-lily, theNelumbium, and remembered Alph. de Candolle’s remarks on this plant, I thought that its distribution
must remain quite inexplicable ; but Audubon statesthat he found the seeds of the great southern waterlily (probably, according to Dr. Hooker, the Nelumbium
luteum) in a heron’s stomach ; although I do not know
the fact, yet analogy makes me believe that a heron
flying to another pond and getting a hearty meal offish, would probably reject from its stomach a pellet
containing the seeds of the Nelumbium undigested;or the seeds might be dropped by the bird whilst
feeding its young, in the same way as fish are known
sometimes to be dropped.
In considering these several means of distribution,

388

GEOGRAPHICAL DISTRIBUTION.

CHAP. XII.

it should be remembered that when a pond or stream
is first formed, for instance, on a rising islet, it will beunoccupied ; and a single seed or egg will have a goodchance of succeeding. Slthough there will always be a
struggle for life between the individuals of the species,
however few, already occupying a j y pond, yet as thenumber of kinds is small, compared with those on theland, the competition will probably be less severe
between aquatic than between terrestrial species ; consequently an intruder from the waters of a foreigncountry, would have a better chance of seizing on aplace, than in the case of terrestrial colonists. We
should, also, remember that some, perhaps many, freshwater productions are low in the scale of nature, andthat we have reason to believe that such low beingschange or become modified less quickly than the high ;and this will give longer time than the average for the
migration of the same aquatic species. We should notforget the probability of many species having formerly
ranged as continuously as fresh-water productions evercan range, over immense areas, and having subsequently
become extinct in intermediate regions. But the wide
distribution of fresh-water plants and of the lower
animals, whether retaining the same identical form
or in some degree modified, I believe mainly depends
on the wide dispersal of their seeds and eggs by animals,
more especially by fresh-water birds, which have large
powers of flight, and naturally travel from one toanother and often distant piece of water. Nature, likea careful gardener, thus takes her seeds from a bed ofa particular nature, and drops them in another equallywell fitted for them.
On the Inha6itants of Oceanic ~ s l a n d s . - ~ e now
come to the last of the three classes of facts, which I

CHAP. XII.

OCEANIC ISLANDS.

38 9

have selected as presenting the greatest amount ofdifficulty, on the view that all the individuals both of
the same and of allied species have descended from asingle parent ; and therefore have all proceeded from acommon birthplace, notwithstanding that in the courseof time they have come to inhabit distant points of the
globe. I have already stated that I cannot honestly
admit Forbes's view on continental extensions, which,if legitimately followed out, would lead to the belief
that within the recent period all existing islands havebeen nearly or quite joined to some continent. This view
would remove many difficulties, but it would not, I
think, explain all the facts in regard t o insular productions. I n the following remarks I shall not confine
myself t o the mere question of dispersal; but shallconsider some other facts, which bear on the truth of
the two theories of independent creation and of descentwith modification.
The species of all kinds which inhabit oceanic islands
are few in number compared with those on equal continental areas : Alph. de Candolle admits this for plants,
and Wollaston for insects. If we look to the large
size and varied stations of New Zealand, extending over
780 miles of latitude, and compare its flowering plants,
only 750 in number, with those on an equal area a t
the Cape of Good Hope or in Australia, we must, I
think, admit that something quite independently of'any difference in physical conditions has caused so greata difference in number. Even the uniform county ofCambridge has 847 plants, and the little island ofAnglesea 764, but a few ferns and a few introduced
plants are included in these numbers, and the comparison in some other respects is not quite fair. We
have evidence that the barren island of Ascension
aboriginally possessed under half-a-dozen flowering

390

GEOGRAPHICAL DISTRIBUTION.

CHAP.XII.

plants ; yet many have become naturalised on it,
as they have on New Zealand and on every otheroceanic island which can be named. I n St. Helena
there is reason to believe that the naturalised plants
and animals have nearly or quite exterminated manynative productions. He who admits the doctrine of
the creation of each separate species, will have toadmit, that a sufficient number of the best adapted
plants and animals have not been created on oceanicislands ; for man has unintentionally stocked them fromvarious sources far more fully and perfectly than has
nature.
Although in oceanic islands the number of kindsof inhabitants is scanty, the proportion of endemic
species (i. e. those found nowhere else in the world)
is often extremely large. If we compare, for instance,
the number of the endemic land-shells in Madeira, orof the endemic birds in the Galapagos Archipelago, with
the number found on any continent, and then compare
the area of the islands with that of t.he continent, weshall see that this is true. This fact might have been
expected on my theory, for, as already explained, species occasionally arriving after long intervals in a new
and isolated district, and having to compete with new
associates, will be eminently liable to modification, andwill often produce groups of modified descendants. But
it by no means follows, that, because in an island nearly
all the species of one class are peculiar, those of anotherclass, or of another section of the same class, are peculiar ; and this difference seems to depend on the species
which do not become modified having immigrated withfacility and in a body, so that their mutual relations
have not been much disturbed. Thus in the Galapagos
Islands nearly every land-bird, but only two out of theeleven marine birds, are peculiar ; and it is obvious that

CHAP. XIT.

OCEANIC ISLANDS.

39 1

marine birds could arrive at these islands more easily thanland-birds. Bermuda, on the other hand, which lies atabout the same distance from North America as theGalapagos Islands do from South America, and which
has a very peculiar soil, does not possess one endemic
land bird; and we know from Mr. J. M. Jones’s admirable account of Bermuda, that very many North
American birds, during their great annual migrations,visit either periodically or occasionally this island.
Madeira does not possess one peculiar bird, and many
European and African birds are almost every year blomn
there, as I am informed by Mr. E. V. Harcourt. So thatthese two islands of Bermuda and Madeira have beenstocked by birds, which for long ages have struggled
together in their former homes, and have become mutually adapted to each other ; and when settled in their
new homes, each kind will have been kept by the others
to their proper places and habits, and will consequently
have been little liable to modification. Madeira, again,
is inhabited by a wonderful number of peculiar landshells, whereas not one species of sea-shell is confined toits shores: now, though we do not know how sea-shells
are dispersed, yet we can see that their eggs or larvae,
perhaps attached to seaweed or floating timber, or to
the feet of wading-birds, might be transported far moreeasily than land-shells, across three or four hundred
miles of open sea. The different orders of insects in
Madeira apparently present analogous facts.
Oceanic islands are sometimes deficient in certainclasses, and their places are apparently occupied by
the other inhabitants ; in the Galapagos Islands reptiles,
and in New Zealand gigantic wingless birds, take theplace of mammals. In the plants of the Galapagos
Islands, Dr. Hooker has shown that the proportional
numbers of the different orders are very different from

392

GEOGRAPHICAL DISTRIBUTION.

CHAP. XII.

what they are elsewhere. Such cases are generally accounted for by the physical conditions of the islands ;but this explanation seems to me not a little doubtful.
Facility of immigration, I believe, has been a t least as
important as the nature of the conditions.
Many remarkable little facts could be given withrespect to the inhabitants of remote islands. For
instance, in certain islands not tenanted by mammals,
some of the endemic plants have beautifully hooked
seeds; yet few relations are more striking than theadaptation of hooked seeds for transportal by the wool
and fur of quadrupeds. This case presents no difficultyon my view, for a hooked seed might be traneported to
an island by some other means ; and the plant thenbecoming slightly modified, but still retaining its hooked
seeds, would form an endemic species, having as uselessan appendage as any rudimentary organ,-for instance,as the shrivelled wings under the soldered elytra ofmany insular beetles. Again, islands often possess trees
or bushes belonging to orders which elsewhere include
only herbaceous species; now trees, as Alph. de Candolle has shown, generally have, whatever the cause
may be, confined ranges. Hence trees would be little
likely to reach distant oceanic islands; and an herbaceous plant, though it would have no chance of
successfully competing in stature with a fully developed tree, when established on an island and having
to compete with herbaceous plants alone, might readily
gain an advantage by growing taller and taller andovertopping the other plants. If so, natural selection
would often tend to add to the stature of herbaceous
plants when growing on an island, to whatever order
they belonged, and thus convert them first into bushes
and ultimately into trees.
With respect to the absence of whole orders on

CEAP. XII.

OCEANIC ISLANDS.

393

oceanic islands, Boiy St. Vincent long ago remarked
that Batrachians (frogs, toads, newts) have never beenfound on any of the many islands with which the great
oceans are studded. I have taken pains to verify thisassertion, and I have found it strictly true. I have,
however, been assured that a frog exists on the mountains of the great island of New Zealand ; but I suspectthat this exception (if the information be correct) may
be explained through glacial agency. This generalabsence of frogs, toads, and newts on so many oceanicislands cannot be accounted for by their physical conditions ; indeed it seems that islands are peculiarly wellfitted for these animals ; for frogs have been introduced
into Madeira, the Azores, and Mauritius, and havemultiplied so as to become a nuisance. But as theseanimals and their spawn are known to be immediately
killed by sea-water, on my view we can see that
there would be great difficulty in their transportal
across the sea, and therefore why they do not exist on
any oceanic island. But why, on the theory of creation,
they should not have been created there, it would bevery difficult to explain.
Mammals offer another and similar case. I havecarefully searched the oldest voyages, but have notfinished my search; as yet I have not found a single
instance, free from doubt, of a terrestrial mammal(excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from acontinent or great continental island ; and many islands
situated at a much less distance are equally barren.
The Falkland Islands, which are inhabited by a wolflike fox, come nearest to an exception ; but this groupcannot be considered as oceanic, as it lies on a bank connected with the mainland ; moreover, icebergs formerly
brought boulders t o its western shores, and they may

394

GEOGRAPHICAL DISTRIBUTION.

CHAP.XII.

have formerly transported foxes, as so frequently now
happens in the arctic regions. Yet it cannot be said
that small islands will not support small mammals, forthey occur in many parts of the world on verysmall islands, if close to a continent; and hardly an
island can be named on which our smaller quadrupeds
have not become naturalised and greatly multiplied.
It cannot be said, on the ordinary view of creation,that there has not been time for the creation of mammals ; many volcanic islands are sufficiently ancient,as shown by the stupendous degradation which they
have suffered and by their tertiary strata: there has
also been time for the production of endemic species
belonging to other classes; and on continents it isthought that mammals appear and disappear at aquicker rate than other and lower animals. Thoughterrestrial mammals do not occur on oceanic islands,
aerial mammals do occur on almost every island. New
Zealand possesses two bats found nowhere else in theworld : Norfolk Island, the Viti Archipelago, the Bonin
Islands, the Caroline and Marianne Archipelagoes, andNauritius, all possess their peculiar bats. Why, it may
be asked, has the supposed creative force produced
bats and no other mammals on remote islands? On
my view this question can easily be answered ; for no
terrestrial mammal can be transported across a wide
space of sea, but bats can fly across. Bats have been
seen wandering by day far over the Atlantic Ocean;and two North American species either regularly oroccasionally visit Bermuda, at the distance of 600 miles
from the mainland. I hear from Mr. Tomes, who has
specially studied this family, that many of the same
species have enormous ranges, and are found on continents and on far distant islands. Hence we have only
to suppose that such wandering species have been modi-

CHAP. XII.

OCEANIC ISLANDS.

395

fied through natural selection in their new homes inrelation to their new position, and we can understandthe presence of endemic bats on islands, with the absence of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the remoteness of islands from continents, there
is also a relation, to a certain extent independent of
distance, between the depth of the sea separating an
island from the neighbouring mainland, and the presence in both of the same mammiferous species or ofallied species in a more or less modified condition. Mr.
Windsor Earl has made some striking observations on
this head in regard to the great Malay Archipelago,which is traversed near Celebes by a space of deep
ocean; and this space separates two widely distinct
mammalian faunas. On either side the islands are
situated on moderately deep submarine banks, and they
are inhabited by closely allied or identical quadrupeds.
No doubt some few anomalies occur in this great archipelago, and there is much difficulty in forming a judgment in some cases owing to the probable naturalisationof certain mammals through man’s agency; but weshall soon have much light thrown on the natural
history of this archipelago by the admirable zeal andresearches of Mr. Wallace. I have not as yet had time tofollow up this subject in all other quarters of the world ;but as far as I have gone, the relation generally holds
good. We see Britain separated by a shallow channel
from Europe, and the mammals are the same on bothsides ; we meet with analogous facts on many islands
separated by similar channels from Australia. The
West Indian Islands stand on a deeply submerged bank,nearly 1000 fathoms in depth, and here we find American
forms, but the species and even the genera are distinct.
As the amount of modification in all cases depends to396

GEOGRAPHICAL DISTRIBUTION.

CHAP.XII.

a certain degree on the lapse of time, and as duringchanges of level it is obvious that islands separated byshallow channels are more likely to have been continuously united within a recent period to the mainland than islands separated by deeper channels, we canunderstand the frequent relation between the depth of
the sea and the degree of affinity of the mammalian
inhabitants of islands with those of a neighbouring continent,-an inexplicable relation on the view of independent acts of creation.
All the foregoing remarks on the inhabitants ofoceanic islands,-namely, the scarcity of kinds-the
richness in endemic forms in particular classes or sections of classes,-the absence of whole groups, as ofbatrachians, and of terrestrial mammals notwithstanding
the presence of aerial bats,-the singular proportions ofcertain orders of plants,-herbaceous forms having beendeveloped into trees, &c.,-seem to me t o accord betterwith the view of occasional means of transport having
been largely efficient in the long course of time, thanwith the view of all our oceanic islands having been
formerly connected by continuous land with the nearest
continent ; for on this latter view the migration would
probably have been more complete ; and if modification be admitted, all the forms of life would havebeen more equally modified, in accordance with theparamount importance of the relation of organism toorganism.
I do not deny that there are many and grave difficulties in understanding how several of the inhabitantsof the more remote islands, whether still retaining thesame specific form or modified since their arrival, could
have reached their present homes. But the probabilityof many islands having existed as halting-places, ofwhich not a wreck now remains, must not be over-CHAP. XII,OCEANIC ISLANDS.

397

looked. I will here give a single instance of one of
the cases of dBculty. Almost all oceanic islands,
even the most isolated and smallest, are inhabited byland-shells, generally by endemic species, but sometimes
by species found elsewhere. Dr. Aug. A. Gould has
given several iiiteresting cases in regard to the landshells of the islands of the Pacific. Now it is notoriousthat land-shells are very easily killed by salt; theireggs, at least such as I have tried, sink in sea-waterand are killed by it. Yet there must be, on my view,
some unknown, but highly efficient means for their transportal. Would the just-hatched young occasionallycrawl on and adhere to the feet of birds roosting on the
ground, and thus get transported? It occurred tome that land-shells, when hybernat,ing and having amembranous diaphragm over the mouth of the shell,
might be floated in chinks of drifted timber acrossmoderately wide arms of the sea. And I found thatseveral species did in this state withstand uninjured an
immersion in sea-water during seven days : one of theseshells was the Helix pomatia, and after it had again
hybernated I put it in sea-water for twenty days,
and it perfectly recovered. As this species has athick calcareous operculum, I removed it, and when it
had formed a new membranous one, I immersed it for
fourteen days in sea-water, and it recovered andcrawled away: but more experiments are wanted on
this head.
The most striking and important fact for us in regardt o the inhabitants of islands, is their affinity to those of
the nearest mainland, without being actually the same
species. Numerous instances could be given of thisfact. I will give only one, that of the Galapagos
Archipelago, situated under the equator, between 500
and 600 miles from the shores of South America. Here398

GEOGRAPHICAL DISTRIBUTION.

CHAP. XII.

almost every product of the land and water bears theunmistakeable stamp of the American continent. There
are twenty-six land birds, and twenty-five of these areranked by Mr. Gould as distinct species, supposed tohave been created here ; yet the close affinity of mostof these birds to American species in every character,in their habits, gestures, and tones of voice, was manifest. So it is with the other animals, and with nearly
all the plants, as shown by Jlr. Hooker in his admirable
memoir on the Flora of this archipelago. The naturalist, looking at the inhabitants of these volcanicislands in the Pacific, distant several hundred miles
from the continent, yet feels that he is standing on
American land. Why should this be S O ? why should
the species which are supposed to have been created in
the Galapagos Archipelago, and nowhere else, bear soplain a. stamp of affinity to those created in America?
There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate,or in the proportions in which the several classes areassociated together, which resembles closely the conditions of the South American coast : in fact there isa considerable dissimilarity in all these respects. On
the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in climate,height, and size of the islands, between the Galapagos
and Cape de Verde Archipelagos: but what an entire
and absolute difference in their inhabitants! The inhabitants of the Cape de Verde Islands are related tothose of Africa, like those of the Galapagos to America.
I believe this grand fact can receive no sort of explanation on the ordinary view of independent creation ;whereas on the view here maintained, it is obviousthat the Galapagos Islands would be likely t o receive
colonists, whether by occasional means of transport orCHAP.XII.

OCEANIC ISLANDS.

399

by foi-merly continuous land, from America ; and theCape de Verde Islands from Africa; and that suchcolonists would be liable to modification ;-the principle
of inheritance still betraying their original birthplace.
Many analogous facts could be given : indeed it is analmost universal rule that the endemic productions ofislands are related to those of the nearest continent, orof other near islands. The exceptions are few, andmost of them can be explained. Thus the plants of
Kerguelen Land, though standing nearer to Africa thanto America, are related, and that very closely, as weknow from Dr. Hooker’s account, to those of America :but on the view that this island has been mainly stocked
by seeds brought with earth and stones on icebergs,
drifted by the prevailing currents, this anomaly disappears. New Zealand in its endemic plants is much
more closely related to Australia, the nearest mainland,
than to any other region: and this is what might havebeen expected ; but it is also plainly related to SouthAmerica, which, although the next nearest continent,
is so enormously remote, that the fact becomes ananomaly. But this difficulty almost disappears on theview that both New Zealand, South America, andother southern lands were long ago partially stockedfrom a nearly intermediate though distant point, namelyfrom the antarctic islands, when they were clothedwith vegetation, before the commencement of the Glacial period. The affinity, which, though feeble, I am
assured by Dr. Hooker is real, between the flora of
the south-western corner of Australia and of the Capeof Good Hope, is a far more remarkable case, and is
at present inexplicable : but this affinity is confined tothe plants, and will, I do not doubt, be some day explained.
The law which causes the inhabitants of an archi-

400

GEOGRAPHICAL DISTRIBUTION.

CHAP. XII.

pelago, though specifically distinct, to be closely allied
to those of the nearest continent, we sometimes see
displayed on a small scale, yet in a most interestingmanner, within the limits of the same archipelago.
Thus the several islands of the Galapagos Archipelago
are tenanted, as I have elsewhere shown, in a quite
marvellous manner, by very closely related species ;so that the inhabitants of each separate island, thoughmostly distinct, are related in an incomparably closer
degree to each other than to the inhabitants of any
other part of the world. And this is just what might
have been expected on my view, for the islands are
situated so near each other that they would almost
certainly receive immigrants from the same original
source, or from each other. But this dissimilarity
between the endemic inhabitants of the islands may
be used as an argument against my views ; for it may
be asked, how has it happened in the several islands
situated within sight of each other, having the samegeological nature, the same height, climate, &c., thatmany of the immigrants should have been differently
modified, though only in a small degree. This long
appeared to me a great difficulty: but it arises inchief part from the deeply-seated error of considering
the physical conditions of a country as the most important for its inhabitants ; whereas it cannot, I think,
be disputed that the nature of the other inhabitants,
with which each has to compete, is at least as important, and generally a far more important element of
success. Now if we look t o those inhabitants of theGalapagos Archipelago which are found in other partsof the world (laying on one side for the moment theendemic species, which cannot be here fairly included,
as we are considering how they have come to be modified since their arrival), we find a considerable amountCHAP. XII.

OCEANIC ISLANDS.

401

of difference in the several islands. This difference
might indeed have been expected on the view of theislands having been stocked by occasional means oftiansport-a seed, for instance, of one plant having
been brought to one island, and that of another plantt o another island. Hence when in former times animmigrant settled on any one or more of the islands, orwhen it subsequently spread from one island to another,
it would undoubtedly be exposed to different conditionsof life in the different islands, for it would have tocompete with different sets of organisms : a plant, forinstance, would find the best-fitted ground more perfectly occupied by distinct plants in one island thanin another, and it would be exposed to the attacks ofsomewhat different enemies. If then it varied, natural
selection would probably favour different varieties inthe different islands. Some species, however, might
spread and yet retain the same character throughout
the group, just as we see on continents some species
spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago, and in a lesser degree in someanalogous instances, is that the new species formed in
the separate islands have not quickly spread to the otherislands. But the islands, though in sight of each other,are separated by deep arms of the sea, in most cases
wider than the British Channel, and there is no reason
to suppose that they have at any former period been
continuously united. The currents of the sea are rapid
and sweep across the archipelago, and gales of wind
are extraordinarily rare; so that the islands are far
more effectually separated from each other than they
appear to be on a map. Nevertheless a good many
species, both those found in other parts of the world
and those confined to the archipelago, are common to402

GEOGRAPHICAL DISTRIBUTION.

CHAP.XII.

the several islands, and we may infer from certain facts
that these have probably spread from some one island
to the others. But we often take, I think, an erroneous view of the probability of closely allied species
invading each other’s territory, when put into freeintercommunication. Undoubtedly if one species has
any advantage whatever over another, it will in avery brief time wholly or in part supplant it; but if
both are equally well fitted for their own places innature, both probably will hold their own places andkeep separate for almost any length of time. Being
familiar with the fact that many species, naturalised
through man’s agency, have spread with astonishingrapidity over new countries, we are apt to infer thatmost species would thus spread ; but we should remember that the forms which become naturalised in new
countries are not generally closely allied to the aboriginal
inhabitants, but are very distinct species, belonging in alarge proportion of cases, as shown by Alph. de Candolle,
to distinct genera. I n the GalapagosArchipelago, manyeven of the birds, though so well adapted for flying
from island to island, are distinct on each ; thus there
are three closely-allied species of mocking-thrush, eachconfined to its own island. Now let us suppose themocking-thrush of Chatham Island to be blown toCharles Island, which has its own mocking-thrush : whyshould it succeed in establishing itself there ? We may
safely infer that Charles Island is well stocked with its
own species, for annually more eggs are laid there
than can possibly be reared ; and we may infer that themocking-thrush peculiar to Charles Island is at least as
well fitted for its home as ie the species peculiar to
Chatham Island. Sir C. Lye11 and Mr. Wollaston havecommunicated to me a remarkable fact bearing on thia
subject ; namely, that Madeira and the adjoining islet of

CHAP.XII.

OCEANIC ISLANDS.

403

Porto Santo possess many distinct but representativeland-shells, some of which live in crevices of stone ; andalthough large quantities of stone are annually transported from Porto Santo to Madeira, yet this latterisland has not become colonised by the Porto Santo
species : nevertheless both islands have been colonisedby some European land-shells, which no doubt had someadvantage over the indigenous species. From these
considerations I think we need not greatly marvel atthe endemic and representative species, which inhabif
the several islands of the Galapagos Archipelago, nothaving universally spread from island to island. In
many other instances, as in the several districts of thesame continent, pre-occupation has probably played an
important part in checking the commingling of species
under the same conditions of life. Thus, the south-eastand south-west corners of Australia have nearly thesame physical conditions, and are united by continuous
land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants.
The principle which determines the general characterof the fauna and flora of oceanic islands, namely, thatthe inhabitants, when not identically the same, yet areplainly related to the inhabitants of that region whencecolonists could most readily have been derived,-thecolonists having been subsequently mod3ed and better
fitted to their new homes,-is of the widest application throughout nature. We see this on every mountain, in every lake and marsh. For Alpine species,
excepting in so far as the same forms, chiefly of plants,
have spread widely throughout the world during therecent Glacial epoch, are related to those of the surrounding lowlands ;-thus we have in South America,
Alpine humming-birds, Alpine rodents, Alpine plants,&c., all of strictly American forms, and it is obvious

404

GEOGRAPHICAL DISTRIBUTIOX.

CHAP. XI:.

that a mountain, as it became slowly upheaved, would
naturally be colonised from the surrounding lowlands.
So it is with the inhabitants of lakes and marshes,
excepting in so far as great facility of transport hasgiven the same general forms to the whole world. We
see this same principle in the blind animals inhabiting
the caves of America and of Europe. Other analogous
facts could be given. And it will, I believe, be universally found to be true, that wherever in two regions,
let them be ever so distant, many closely allied or representative species occur, there will likewise be foundsome identical species, showing, in accordance with theforegoing view, that a t some former period there has
been intercommunication or migration between the tworegions. And wherever many closely-allied species
occur, there will be found many forms which somenaturalists rank as distinct species, and some as varieties ; these doubtful forms showing us the steps in theprocess of modification.
This relation between the power and extent of migration of a species, either a t the present time or at someformer period under different physical conditions, andthe existence a t remote points of the world of otherspecies allied to it, is shown in another and moregeneral way. Mr. Gould remarked to me long ago, thatin those genera of birds which range over the world,many of the species have very wide ranges. I canhardly doubt that this rule is generally true, though it
would be difficult to prove it. Amongst mammals, we
see it strikingly displayed in Bats, and in a lesser degreein the Felidae and Canide. We see it, if we compare
the distribution of butterflies and beetles. So it is withmost fresh-water productions, in which so many genera
range over the world, and many individual species haveenormous ranges. It is not meant that in world-

CHAP. XI[.

OCEANIC ISLANDS.

405

ranging genera all the species have a wide range, oreven that they have on an average a wide range ; but
only that some of the species range very widely ; for thefacility with which widely-ranging species vary and giverise t o new forms will largely determine their average
range. For instance, two varieties of the same species
inhabit America and Europe, and the species thus has
an immense range ; but, if the variation had been a little
greater, the two varieties would have been ranked as distinct species, and the common range would have been
greatly reduccd. Still less is it meant, that a species
which apparently has the capacity of crossing barriers
and ranging widely, as in the case of certain powerfullywinged birds, will necessarily range widely; for weshould never forget that to range widely implies not
only the power of crossing barriers, but the more important power of being victorious in distant lands inthe struggle for life with foreign associates. But
on the view of all the species of a genus having dedescended from a single parent, though now distributed
to the most remote points of the world, we ought tofind, and I believe as a general rule we do find, thatsome at least of the species range very widely ; for it isnecessary that the unmodified parent should range
widely, undergoing modification during its diffusion, andshould place itself under diverse conditions favourable
for the conversion of its offspring, firstly into new varieties and ultimately into new species.
In considering the wide distribution of certain genera,
we should bear in mind that some are extremely ancient,and must have branched off from a common parent
at a remote epoch; so that in such cases there will
have been ample time for great climatal and geographicalchanges and for accidents of transport; and consequently for the migration of some of the species into all406

GEOGRAPHICAL DISTRIBUTION.

CHAP. XI.

quarters of the world, where they may have becomeslightly modified in relation to their new conditions.
There is, also, some reason to believe from geologicalevidence that organisms low in the scale within eachgreat class, generally change a t a slower rate than thehigher forms ;and consequently the lower forms will have
had a better chance of ranging widely and of still retaining the same specific character. This fact, together
with the seeds and eggs of many low forms being very
minute and better fitted for distant transportation, probably accounts for a law which has long been observed,
and which has lately been admirably discussed by Alph. de
Candolle in regard to plants, namely, that the lower anygroup of organisms is, the more widely it is apt to range.
The relations just discussed,-namely, low and slowlychanging organisms ranging more widely than thehigh,-some of the species of widely-ranging genera
themselves ranging widely,-such facts, as alpine, lacustrine, and marsh productions being related (with the
exceptions before specified) to those on the surrounding
low lands and dry lands, though these stations are so
different-the very close relation of the distinct species
which inhabit the islets of the same archipelago,-and
especially the striking relation of the inhabitants ofeach whole archipelago or island to those of the nearest
mainland,-are, I think, utterly inexplicable on theordinary view of the independent creation of each species, but are explicable on the view of colonisationfrom the nearest and readiest source, together with thesubsequent modification and better adaptation of theIcolonists to their new homes.

Summary of last and present Chapters.-In
thesechapters I have endeavoured to show, that if we makedue allowance for our ignorance of the full effects of all

CHAP. XI.

SUMMARY.

407

the changes of climate and of the level of the land,
which have certainly occurred within the recent period,and of other similar changes which may have occurred
within the same period; if we remember how profoundly ignorant we are with respect to the manyand curious means of occasional transport,-a subject
which has hardly ever been properly experimentisedon; if we bear in mind how often a species may haveranged continuously over a wide area, and then havebecome extinct in the intermediate tracts, I think the
dif6culties in believing that all the individuals of thesame species, wherever located, have descended from thesame parents, are not insuperable. And we are led tothis conclusion, which has been arrived at by manyna.tiiralists under the designation of single centres ofcreation, by some general considerations, more especiallyfrom the importance of barriers and from the analogicaldistribution of sub-genera, genera, and families.
With respect to the distinct species of the same genus,
which on my theory must have spread from one parentsoilrce; if we make the same allowances as before forour ignorance, and remember that some forms of lifechange most slowly, enormous periods of time being
thus granted for their migration, I do not think that the
dif6culties are insuperable; though they often are inthis case, and in that of the individuals of the same species, extremely grave.
As exemplifying the effects of climatal changes on
distribution, I have attempted to show how important
has been the influence of the modern Glacial period,
which I am fully convinced simultaneously affected thewhole world, or at least great meridional belts. As
showing how diversified are the means of occasionaltransport, I have discussed at some little length themeans of dispersal of fresh-water productions.

408

GEOURAPHICAL DISTRIBUTION.

CHAP.XII.

If the difficulties be not insuperable in admittingthat in the long course of time the individuals of thesame species, and likewise of allied species, have proceeded from some one source ; then I think all the grandleading facts of geographical distribution are explicableon the theory of migration (generally of the more dominant forms of life), together with subsequent modification and the multiplication of new forms. We canthus understand the high importance of barriers, whetherof land or water, which separate our several zoologicaland botanical provinces. We can thus understand the
localisation of sub-genera, genera, and families ; and how
it is that under different latitudes, for instance in SouthAmerica, the inhabitants of the plains and mountains, ofthe forests, marshes, and deserts, are in so mysterious
a manner linked together by affinity, and are likewise
linked to the extinct beings which formerly inhabited thesame continent. Bearing in mind that the mutual relations of organism to organism are of the highest importance, we can see why two areas having nearly the same
physical conditions should often be inhabited by very
different forms of life ;for according to the length of time
which has elapsed since new inhabitants entered one
region ; according to the nature of the communication
which allowed certain forms and not others to enter, either
in greater or lesser numbers ; according or not, as thosewhich entered happened to come in more or less directcompetition with each other and with the aborigines ;and according as the immigrants were capable of varying more or less rapidly, there would ensue in different
regions, independently of their physical conditions, 3nitely diversified conditions of life,-there would be analmost endless amount of organic action and reaction,and we should find, as we do find, some groups of beings
greatly, and some only slightly modified,-some deve-CHAP. XII.

SUNMARY.

409

loped in great force, some existing in scanty number^in the different great geographical provinces of theworld.
On these same principles, we can understand, as I
have endeavoured to show, why oceanic islands shouldhave few inhabitants, but of these a great number
should be endemic or peculiar ; and why, in relation to
the means of migration, one group of beings, even within
the same class, should have all its species endemic, andanother group should have all its species common toother quarters of the world. We can see why wholegroups of organisms, as batrachians and terrestrial mammals, should be absent from oceanic islands, whilst themost isolated islands possess their own peculiar species ofaerial mammals or bats. We can see why there should
be some relation between the presence of mammals, ina more or less modified condition, and the depth of
the sea between an island and the mainland. W e canclearly see why all the inhabitants of an archipelago,
though specifically distinct on the several islets, should
be closely related to each other, and likewise be related,
but less closely, to those of the nearest continent orother source whence immigrants were probably derived
We can see why in two areas, however distant from eachother, there should be a correlation, in the presence ofidentical species, of varieties, of doubtful species, and of
distinct but representative species,
As the late Edward Forbes often insisted, there is a
striking parallelism in the laws of life throughout timeand space : the laws governing the succession of forms inpast t,imes being nearly the same with those governingat the present time the differences in different areas.
We see this in many facts. The endurance of eachspecies and group of species is continuous in time ; forthe exceptions to the rule are so few, that they may

410

GEOGRAPHICAL DISTBIBUTION.

CHAP.XIIS

fairly be attributed to our not having as yet discoveredin an intermediate deposit the forms which are thereinabsent, but which occur above and below : so in space,it certainly is the general rule that the area inhabited by a single species, or by a group of species, iS
continuous; and the exceptions, which are not rare,
may, as I have attempted to show, be accounted for by
migration at some former period under different conditions or by occasional means of transport, and by the
species having become extinct in the intermediate tracts.
Both in time and space, species and groups of species
have their points of maximum development. Groups ofspecies, belonging either to a certain period of time, orto a certain area, are often characterised by trifling characters in common, as of sculpture or colour. I n looking to the long succession of ages, as in now looking to
distant provinces throughout the world, we find thatsome organisms differ little, whilst others belonging to adifferent class, or to a different order, or even only to adifferent family of the same order, differ greatly. I nboth time and space the lower members of each classgenerally change less than the higher; but there are
in both cases marked exceptions to the rule. On mytheory these several relations throughout time and space
are intelligible ; for whether we look to the forms of life
which have changed during successive ages within thesame quarter of the world, or to those which havechanged after having migrated into distant quarters, inboth cases the forms within each class have been connected by the same bond of ordinary generation; andthe more nearly any two forms are related in blood, thenemer they will generally stand to each other in timeand space ; in both cases the laws of variation have been
the same, and modifications have been accumulated by
the same power of natural selection.

INDEX,ABERRAYT.

A.
ABERRANTgroups, 429.
Abyssinia, plants o< 375.
Acclirnatisation, 139.
Affiities of extinct species, 329.
-of organic beings, 41 1 ,
Agassiz on Amblyopsis, 139.
on groups of species suddenly
appeirinp, 302, 305.
on embryo10,vical succession, 338.
on the glacial period, 366.
on embryological characters, 418.
011 the embryos of vertebrata,439.
on parallelism of embrpologicd
development .and geological S U C C C ~sion, 449.
Alga of Kew Zealand, 376.
Alligators, males, fighting, 88.
Amblyopsis, blind fish, 139.
America, North, productions allied tothose of Enrope, 371.
-,
boulders and glaciers of,373.
-, South, no modern furmations onwest coast, 290.
Ammonites, sudden extinction of, 321.
Anagallis, sterility of, 247.
Analogy of variations, 159.
Ancylus, 386.
Animals, not domesticated from being
variable, 17.
-, domestic, descended !?om seve&lstocks, 19.
-,
acclimatisation OF, 141.
of Australia, 116.
with thicker fur in cold climates,133.
-, blind, in caves, 137.
-, extinct, of Australia, 339.
Anomma, 240.
Antarctic islands, ancient flora of, 399.
Antirrhinum, 161.
Ants attending aphides, 21 1.
-, slave-making instinct, 219.

-

-

BEES.

Ants, neuter, structure of, 236.
Aphides attended by ants, 211.
Aphis, development of, 442.
Apteryx, 182.
Arab horsps, 35.
Ado-Caspian Sea, 339.
Archiac, 3f. de, on the succession of
species, 325.
Artichoke, Jerusalem, 142.
Ascension, plants of, 389.
Asclepias, pollen of, 193.
Asparagus, 359.
Aspkmp, 41 7.
Asses, striped, 163.
Ateuchus, 135.
Budubon on hiibits of frigate-bird, 185.
on variation in birds’-nests, 212.
on heron eating seeds, 387.
Australia, animals ot, 116.
-, dogs of, 215.
-, extinct animals of, 339.
-, European plants in, 375.
Azara on flies destroying cattle, 72.
Azores, flora of, 363.

-

B.
Babington, Mr., on British plants, 48.
Balancement of growth, 147.
Bamboo with hooks, 197.
Barberry, flowers of, 98.
Barrande, M., on Silurian colonies,

313.

-on the succession of species, 325.
-on parallelism of palaeozoic formations, 328.
- on a h i t i e s of ancient 6peCieS,
330.

Barriers, importance of, 347.
Batmcbims on islands, 393.
Bats, how structure acquired, 180.
-, distribution of, 394.
Bear, catching water-insects, 184.
Bee, sting of, 202.
-,
queen, killing rivals, 202.
Bees fertilising flowers, 73.

492

INDEX.
~BEES.

CLIMATE.

Bees, hive, not sucking the red clover,95.
,-, cell-making instinct, 224.
-, humble, cells of, 225.
-, parasitic, 218.
Beetles, wingless, in Madeira, 135.
with deficient tarsi, 135.
Beutham, Mr., on British plants, 48.
, on classification, 419.
Berkeley, Mr., on seeds in salt-water,
358.
Bermuda, birds of, 391.
Birds acquiring fear, 212.
annually cross the Atlantic, 364.
-, coloor of, on continents, 132.
-, fossil, in caYes of Brazil, 339.
of Madeira, Bermuda, and GalaPdgOS, 390.
, song of males, 89.
-transporting
seeds, 361.
-, waders, 386.
-, wingless, 134, 182.
-, with traces of embryonic teeth,451.
Bizcacha, 349.
-, affinities of, 429.
Bladder for swimming in fish, 190.
Blindness of cave animals, 137.
Blyth, Mr., on distinctness of Indiancattle, 18.
,on striped Hemionns, 163.
, on crossed geese, 253.
Boar, shoulder-pad of, 88.
Borrow, Mr., on the Spanish pointer,
35.
Bory St. Vincent on Batrachians,
393.
Bosquet, M., on fossil Chthamalus,
304.
Boulders, erratic, on the Azores, 363.
Branchia?, 190.
Brent, Mr., on house-tumblers, 214.
, on hawks killing pigeons, 362.
Brewer, Dr., on American cuckoo,
217.
Britain, mammals of, 395.
Bronn on duration of specific forms,
293.
Brown, Rohert, on classification, 414.
Buckman on variation in plants, 10.
Buzareingues on sterility of varieties,
270.

Cabbage, varieties of, crossed, 99.
Calceolaria, 251.
Canary-birds, sterility of hybrids, 252.
Cape de Velde islands, 398.
Cape of Good Hope, plants of, 110,
375.
Carrier-pigeons killed by hawks, 362.
Cassini on flowers of composita?, 145.
Catasetum, 424.
Cats, with blue eyes, deaf, 12.
,variation in habits of, 91.
curling tail when going to spring,201.
Cattle destroying fir-trees, 71.
destroyed by flies in La Plata,
12.
,breeds of, locally extinct, 111.
, fertility of Indian and European
breeds, 254.
Cave, inhabitants of, blind, 137.
Centres of creation, 352.
Cephalopodae, development of, 442.
Cervulus, 253.
Cetacea, teeth and hair, 144.
Ceylon, plants of, 375.
Chalk formation, 322.
Characters, divergence of, 111.
, sexual, variable, 156.
-, adaptive or analogical, 427.
Charlock, 76.
Checks to increase, 67.
,mutual, 71.
Chickens, instinctive tameness of, 216.
Chthamdlina?, 288.
Chthamaius, cretacean species of, 304.
Circumstances favonrable to selection ofdomestic products, 40.
to natural selection, 101.
Cirripedes capable of crossing, 101.
,carapace aborted, 148.
,their ovigerous frena, 192.
,fossil, 304.
-, larva? of, 440.
Classitication, 413.
Clifi, Mr., on the succession of types,
339.
Climate, effects of, in checking increaseof beings, 68.
-, adaptation of, to organisms, 139.

-

-

-

-

-

-

-

C.

-

_
.

-

--

-

_
.

493

INDEX.

CUtlll'hS.

DUCK.

Cobites, intestine of, 190.
Cockroach, 76.
Collections, palsontological, poor, 287.
Colour, influenced by climate, 132.
-,inrelation to attacks by flies,
195.
Columba liria, parent of domestic pigeons, 23.
Colymbetes, 386.
Compensation of growth, 147.
Composite, outer and inner florets of,144.
-, male flowem of, 451.
Conclusion, general, 480.
Conditions, slight changes in, favourable to fertility, 267.
Coot, 185.
Coral-islands, seeds drifted to, 360.
reefs, indicating movements ofearth, 309.
Corn-crake, 185.
Correlation of g r o d h in domestic productions, 11.
of growth, 143, 198.
Cowslip, 49.
Creation, single centres of, 352.
Crinum, 250.
Crosses, reciprocal, 258.
Crossing of domestic animals, importance in altering breeds, 20.
-, advantages of, 96.
unfavourable t o selection, 102.
Crustacea of New Zealand, 376.
Crustacean, blind, 137.
Cryptocerus, 238.
Ctenomys, blind. 137.
Cuckoo, instinct of, 216.
Currants, grafts of, 262.
Currents of sea, rate of, 359.
Cuvier on couditioiis of existence,206.
on fossil monkeys, 303.
-, Fred., on instinct, 208.

De Candolle on struggle for existence,62.
-on umbellifers, 146.
on general atfinities, 430.
-, Alph., on low plants, widelydispersed, 406.
-, -, on widely-ranging plants
being variable, 53.
-, -, on naturalisation, 115.
-, -, on winged seeds, 146.
-, -, on Alpine species suddenly becoming r u e , 175.
-, -, on distribution of plants
with large seeds, 360.
-, on vegetation ofAustralia,
3i9.
-, -, on fresh-water plants, 386.
- -, on insular plants, 389.
Degridation of roast-rocks, 282.
Denudation, rate of, 285.
of oldest rocks. 308.
Development of ancient forms, 336.
Devonian system, 334.
Dianthus, fertility of crosses, 256.
Dirt on feet of birds, 362.
Dispersal, means of, 956.
during glacial period, 365.
Distribution, geographical, 346.
-, means of, 356.
Disuse, effects of, under nature, 134.
Divergence of character, 111.
Division, physiological, of labour, 115.
Dogs, hairless, with imperfect teeth,
12.
descended from several wild
stocks, 18.
-, domestic instincts of, 213.
-, inherited civilisation of, 215.
-, fertility of breeds together, 254.
-,
of crosses, 265.
-, proportions of, when young,444.
Domestication, variation under, 7.
Downing, Mr., on fruit-trees in America, 55.
Downs, North and South, 285.
Dragon-flies, intestines of, 190.
Drift-timber, 960.
Driver-ant, 240.
Drones killed by other bees, 202.
Duck, domestic, wings of, rtduced, 11.
-, logger-headed, 182.

-

-

-

-

D.
Dana, Prof., on blind cave-animals,
139.
-, on relations of crustaceans ofJapan, 372.
, on crustaceans of New Zealand,
376.

-

-

-

-

-

-

-

494

INDEX.

DUCKWEED.

FRUIT-TREES.

Duckweed, 385.
Dugong, affinities of, 414.
Dung-beetles with deficient tawi, 135.
Dyticus, 386.

Fertility of hybrids, 249.
-from slight changes in conditions,E.

Fish, flying, 182.
teleostean, sudden appearanceof, 305.
-eating seeds, 362, 387.
-,fresh-nvater, distribution of, 384.
Fishes, ganoid, now confined to fresh
water, 107.
_.,electric organs of, 192.
-, ganoid, living in fresh water, 321.
of southern hemisphere, 376.
Flight, powers of, how acquired, 182.
Flowers, structure of, in relation tocrossing, 97.
-of composite and umhellifere,l44.
Forbes, E., on colours of shells, 132.
on abrupt range of shells in
depth, 175.
on poorness of paleontologicalcollections, 287.
on continuous succession ofgenera, 316.
on continental extensions, 357.
- on distribution during glacial
period, 366.
on parallelism in t i e and space,

Earl, Mr. W., on the Mnlay Archipelago, 395.
Ears, drooping, in domestic animals,

-,

11.

rudimentary, 454.
Earth, seeds in roots of trees, 361.
Eciton, 238.
Economy of organisation, 147.
Edentata, teeth and hair, 144.
-, fossil species of, 339.
Edwards, Milne, on physiological divisions of labour, 115.
-, on gradations of structure, 194.
-, on embryologid characters,

418.
Eggs, young birds escaping from, 87.
Electric organs, 192.
Elephant, rate of increase, 64.
of glacial period, 141.
Embryology, 439.
Existence, struggle for, 60.
-, conditions of, 206.
Extinction, &s bearing on natural selection, 109.
of domestic varieties, 111 .

---, 317.
Eye, structure of, 187.
-, correction for aberration, 202.
Eyes reduced in moles, 137.

F.
Fabre, M., on parasitic sphex, 218,
Falconer, Dr., on naturaIisation ofplants in India, 65.
on fossil crocodile, 313.
on elephants and mastodons, 334.
and Cautley on mammals of subHimalayan beds, 340.
Falkland Island, wolf of, 393.
Faults, 285.
Faunas, marine, 348.
Fear, instinctive, in birds, 212.
Feet of birds, young molluscs adhering-

to, 985.

267.

-of crossed varieties, 267.
Fir-trees destroyed by cattle, 71.
-,pollen of, 203.
-,

-

-

409.

Forests, changes in, in America, 74.
Formation, Devonian, 334.
Formations, thickness of, in Britain,

-,

284.

intermittent, 290.
Formica rufescens, 219.
sangninea, 219.
-flava, neuter of, 239.
Frena, ovigerous, of cirripedes, 192.
Fresh-water productions, dispersal of,-

383.
Fries on species in large genera being
closely allied to other species, 57.
Frigate-bird, 185.
Frogs on islands, 393.
Fruit-trees, gradual improvement of,37.
-in United States, 85.
-,varieties of, acclimatised in
United States, 142.

495

INDEX.
FUCI.

Fuci, crossed, 258.
Fur, thicker in cold climates, 133.
Furze, 439.
G.
Galapagos Archipelago, birds of, 390.
-, productions of, 398, 400.
Galeopithecus , 181.
Game, increase of, checked by vermin,
68.
Girtner on sterility of hybrids, 247,
255.
-on reciprocal crosses, 258.
on crossed maize and verbascum,270.
-oncomparison of hybrids andmongrels, 272.
Geese, fertility when crossed, 253.
-, upland, 185.
Genealogy important in classification,
425.
Geoffroy St. Hilaire on balancement,147.
on homologous organs, 434.
--,
lsidore, on variability of repeated parts, 149.
--,
on correlation in monstrosities, 11.
-2
on correlation, 144.
-,
on variable parts being often
monstrous, 155.
Geographical distribution, 346.
Geography, ancient, 487.
Geology, future progress of, 487.
-, imperfection of the record, 279.
Giraffe, tail of, 195.
Glacial period, 365.
Gmelin on distribution, 365.
Gnathodon, fossil, 368.
Godwin-Austen, Mr., on the Malay
Archipelago, 299.
Goethe on compensation of growth,147.
Gooseberry, grafts of, 262.
Gould, Dr. A., on land-shells, 397.
-, Mr., on colours of birds, 132.
-, on birds of the Galapagos, 398.
-,ondistribution ofgenera of birds,
404.
Gourds, crossed, 270.
Grafts, capacity of, 261.
Grasses, varieties of, 113.

---

HOOKER.

Gray, Dr. Asa, on trees of UnitedStates, 100.
, on naturalised plants in the
United States, 115.
-, on rarity of intermediate varieties, 176.
,on Alpine plants, 365.
,Dr. J. E., on striped mule, 165.
Grebe, 185.
Groups, aberrant, 429.
Grouse, colours of, 84.
, red, a doubtful species, 49.
Growth, compensation of, 147.
-, correlation of, in domestic products, 11.
-, correlation of, 143.
_.

-

-

H.
Habit, effect of, under domestication,11.
,effect of, under nature, 134.
-, diversified, of same species, 183.
Hair and teeth, correlated, 144.
Harcourt, Mr. E. V., on the birds ofMadeira, 391.
Hartung, M., on boulders in the Azores,
363.
Hazel-nuts, 559.
Hearne on habits of bears, 184.
Heath, changes in vegetation, 72.
Heer, 0..on plants of Madeira, 107.
Helix pomatia, 397.
Helosciadium, 359.
Hemionus, striped, 163.
Herbert, W., on struggle for existence, 62.
, on sterility of hybrids, 249.
Hermaphrodites crossing, 96.
Heron eating seed, 387.
Heron, Sir R., on peacocks, 89.
Heusinger on white animals not poisoned
by certain plants, 12.
Hewitt, Mr., on sterility offirst crosses9
264.
Himalaya, glaciers of, 373.
-, plants of, 375.
Hippeastrum, 250.
Holly-trees, sexes of, 93.
Hollyhock, varieties of, crossed, 271.
Hooker, Dr., on trees of New Zealand,
100.

-

-

496

INDEX.

HOOKER,LARVB,

Hooker, Dr., on acclimatisation of
Himalayan trees, 140.
-, on flowers of umbelliferae, 145.
-, on glaciers of Himalaya, 373.
-, on algae of New Zealand, 376.
-, on vegetation at the base of theHimalaya, 378.
-, on plants of Tierra del Fuego,
374, 378.
-, on Australian plants, 375, 399.
-,
on relations of flora of SouthAmerica. 379.
-, on flora of the Antarctic lands,
381. 399.
-, on the plants of the Galapagos,
391, 398.
Hooks on bamboos, 197.
-to seeds on islands, 392.
Homer, Mr., on the antiquity of Egyptians, 18.
Horns, rudimentary, 454.
Horse, fossil, in La Plata, 318.
Horses destroyed by flies in La Plata,72.
, striped, 163.
-, proportions of, when young, 445.
Horticulturists, selection applied by,
32.
_ ~ .
Huber on cells of bees, 230.
-, P., on reason blended with instinct, 208.
-, on habitual nature of instincts,
208.
-, on slave-making ants, 219.
-, on Melipona domestica, 225.
Humble-bees, cells of, 225.
Hunter, J., on secondary sexual characters, 150.
Hutton, Captain, on crossed geese, 253.
Huxley, Prof., on structure of hermaphrodit.es, 101.
-, on embryological succession, 338.
-, on homologous organs, 438.
-, on the development of aphis, 442.
Hybrids and mongrels compared, 272.
Hybridism, 245.
Hydra, structure of, 190.

Increase, rate of, 63.
Individuals, numbers favourable to selection, 102.
, many, whether simultaneously
created, 356.
Inheritance, laws of, 12.
a t corresponding ages, 14, 86.
Insects, colour of, fitted for habitations,84.
-, sea-side, colours of, 132.
-, blind, in caves, 138.
-, luminoiis, 193.
-, neuter, 236.
Instinct, 207.
Instincts, domestic, 213.
Intercrossing, advantages of, 96.
Islands, oceanic, 388.
Isolation favourable to selection, 104.

I_

I.
Ibla, 148.
Icebergs transporting seeds, 363.

-

-

J.
Japan, productions of, 372.
Java, plants of, 375.
Jones, Mr. J. M., on the birds of Bermuda, 391.
Jussieu on classification, 417.

K.
Kentucky, raves of, 137.
Kerguelm-land, flora of, 381, 399.
Kidney-bean, acclimatisation of, 142.
Kidneys of birds, 144.
Kirby on tarsi deficient in beetles, 135.
Knight, Andrew, on cause of variation,

7.
Kolreuter on the barberry, 98.
-on sterility of hybrids, 247.
on reciprocal crosses, 258.
on crossed varieties of nicotiana,271.
on crossing male and hermaphrodite flowers, 451.

--

Lamarck on adaptive characters, 427.
Land-shells, distribution of, 397.
of Madeira, natnralised, 402.
Languages, classification of, 422.
Lapse, great, of time, 282.
Larva, 440.

-

497

INDEX.
~~~~LAUREL.

MONGRELS.

Laurel, nectar secreted by the leaves
92.
Laws of variation, 131.
Leech, varieties of, 76.
Leguminosa, nectar secreted by glands,
92.
Lepidosiren, 107, 330.
Life, struggle for, 60.
Lingula, Silurian, 306.
Linneus, aphorism of, 413.
Lion, mane of, 88.
, young of, striped, 439.
Lobelia fulgenu, 73, 98.
Lobelia, sterility of crosses, 250.
Loess of the Rhine, 384.
Lowness of structure connected withvariability, 149.
Lowness, related to wide distribution,
406.
Lubbock, Mr., on the nerves of coccus,
46.
Lucas, Dr. P., on inheritance, 12.
-, on resemblance of child to parent,
275.
Lund and Clausen on fossils of Brazil,
339.
Lyell, Sir C., on the struggle for existence, 62.
,on modern changes of the earth,95.
on measure of denudation, 283.
-, on' a carboniferous land-shell,289.
-,on fossil whales, 303.
-,on strata beneath Silorian systern, 307.
--,on the imperfection of the geological record, 310.
-, oil the a p p w a n c e of species,
312.
--, on Barrande's colonies, 313.
-, on tertiary formations of Europe
and North America, 323.
-, on parallelism of tertiary formations, 328.
-, on transport of seeds by icebergs,363.
,on great alternations of climate,382.
, on the distribution of fresh-watershells, 385.
--, on land-shells of Madeira, 402.

Lyell and Dawson on fossilized trees in
Nova Scotia, 296.

L_

-,

--

-

H.
Macleay on analogical characters, 427.
Madeira, plants of, 107.
_- ,beetles of, wingless, 135.
, fossil land-shells of, 339.
-, birds of, 390.
Magpie tame in Norway, 212.
Maize, crossed, 270.
Malay Archipelago compared with Europe, 299.
-, mammals of, 395.
Malpighiaceae, 417.
Mamma, rudimentary, 451.
Mammals, fossil, in secondary formation, 303.
insular, 393.
Man, origin of races of, 199.
Manatee, rudimentary nails of, 454.
Marsupials of $ustralia, 116.
, fossil species of, 339.
Martens, M., experiment on sceds,
360.
Martin, Mr. W. C., on striped mules,
165.
Matteuchi on the electric organs ofrays, 193.
Matthiola, reciprocal crosses of, 258.
Means of dispersal, 356.
Melipona domestica, 225.
Metamorphism of oldest rocks, 305.
Mice destroying bees, 74.
-, acclimatisation of, 141.
Migratioo, bears on first appearance offossils, 296.
Miller, Prof., on the cells of bees,
226.
Mirabilis, crosses of, 258.
Hissel-thrush, 76.
Misseltoe, complex relations of, 3.
Mississippi, rate of deposition a t mouth,
284.
Mocking-thrush of the Galapagos, 402.
Modification of species, how far applicable, 483.
Moles, blind, 137.
Mongrels, fertility and sterility of,
267.
-and
hybrids compared, 272.

-,

-

498

INDEX.
PEAR.

MONKEYS.

Monkeys, fossil, 303.
Monocanthus. 424.
Mons, Van, on the origin of frnit-trees,
29. 39.
Moquin-Tandon on sea-side plants, 132.
Morphology, 434.
Mozart, musical powers of, 209.
Mud, seeds in, 386.
Mules, striped, 165.
Miiller, Dr. F., on Alpine Australianplants, 375.
Murchison, Sir R., on the formationsof Russia, 289.
-,
on azoic formations, 307.
,on extinction, 317.
Mustela vison, 179.
Myanthus, 424.
Myrmecorystus, 238.
Myrmica, eyes of, 240.

-

N.
Nails, rudimentary, 453.
Natural history, future progress of,484.
selection, 80.
system, 413.
Naturalisation of forms distinct fromthe indigenous species, 115.
in New Zealand, 201.
Nautilus, Silurian, 306.
Nectar of plants, 92.
Nectaries, how formed, 92.
Nelnmbium luteum, 387.
Nests, variation in, 212.
Neuter insects, 236.
Newman, Mr., on humble-bees, 74.
New Zealand, productions of, not perfect, 201.
-, naturalised products of, 337.
-, fossil birds of, 339.
-, glacial action in, 373.
,crustaceans of, 376.
-, alga! Of, 376.
-, number of plank of, 389.
-, flora of, 399.
Nicotiana, crossed varieties of, 271.
, certain species very sterile, 257.
Noble, Mr., on fertility of Rhododendron, 251.
Nodules, phosphatic, in azoic rocks,
307.

-

-

-

-

0.

Oak, varieties of, 50.
Onites apelles, 135.
Orchis, pollen of, 193.
Organs of extreme perfection, 186.
-, electric, of fishes, 192.
of little importance, 194.
,homologous, 434.
-, rudiments of, 450.
Ornithorhynchus, 107, 416.
Ostrich not capable of flight, 134.
-, habit of laying eggs together,218.
-, American, two species of, 349.
Otter, habits of, how acquired, 179.
Ouzel, water, 185.
Owen, Prof., on birds not flying, 134.
-, on vegetative repetition, 149.
-, on variable length of arms inourang-outang, 150.
-, on the swim-bladder of fishes,191.
, on electric organs, 192.
-,on fossil horse of La Plata, 319.
-,on relations of ruminants andpachyderms, 329.
-, on fossil birds of New Zealand,339.
-, on succession of types, 339.
-, on affinities of the dugong, 414.
,on homologous organs, 435.
-,on the metamorphosis of cephalopods and spiders, 442.

-

_
.

_
.

P.
Pacific Ocean, faunas of, 348.
Paley on no organ formed to give pain,
201.
Pallas on the fertility of the wild
stocks of domestic animals, 253.
Paraguay, cattle destroyed by flies, 72.
Parasites, 217.
Partridge, dirt on feet, 362.
Parts greatly developed, variable, 150.
-, degrees of utility of, 201.
Parus major, 183.
Passiflora, 251.
Peaches in United States, 85.
Pear, grafts of, 261.

499

UJDEX.
~-~PELARGONIUP.

FLOBINIA.

Pelwgonium, flowers of, 145.
, sterility of, 251.
Pelvis of women, 144.
P e h ia , 145.
period, glacial, 365.
Petrels, habits of, 184.
p h i a n u s , fertility of hybrids, 253.
Pheasant, young, wild, 216.
Philippi on tertiary species in Sicily,
312.
Pidet, Prof., on group of species suadenly appearing, 302, 305.
,on rate of organic change, 313.
, on continuous succession of g enera, 316.
-, on close alliance of fossils in consecutive formations, 335.
-, on embryological surmssion, 338.
Pierce, Mr.,on varieties of wolves, 91.
Pigeons with feathered feet and skinbetween toes, 12.
,breeds described, and origin of,20.
-, breeds of, how produced, 39, 42.
-, tumbler, not being able to get
out of egg, 87.
,revertiug to blue colour, 160.
-, instinct of tumbling, 214.
-, carriers, killed by hawks, 362.
-, young of, 445.
Pistil, rudimentary, 451.
Plants, poisonous, not affecting certain
coloured animals, 12.
-, selection applied to, 32.
-, gradual improvement of, 37.
not improved in barbarous conntries, 38.
destroyed by iasects, 67.
-, in midst of range, have to struggle
with other plants, 77.
-, nectar of, 92.
-, fleshy, on sea-shores, 132.
,fresh-water, distribution of, 386.
,low in scale, widely distributed,
406.
Plumage, laws of change in sexes ofbirds, 89.
Plums in the United States, 85.
Pointer dog, origin of, 35.
-,habitsof, 213.
Poison not affecting certain eoloured
animals, 12.

Poison, similar effect of, on animals andplan$, 484.
Pollen of fir-trees, 203.
Poole, Col., on striped hemionus, 163.
Potamogeton, 387.
Prestwich, Mr., on English and French
eocene formations, 328.
Primrose, 49.
-, sterility of, 247.
Primnla, varieties of, 49.
Proteolepas, 148.
Proteus, 139.
Psychology, future progress of, 488.

-

-

-

-

-

-

--

Q.
Quagga, striped, 165.
duince, grafts of, 261.

B.
Rabbit, disposition of young, 215.
Races, domestic, characten of, 16.
Racehorses, Arab, 35.
--, English, 356.
Ramond on plants of F’yrenees, 368.
Ramsay, Prof., on thickness of the
British formations, 284.
_., on faults, 285.
Ratio of increase, 63.
Rats, supplanting each other, 76.
-, acclimatisation of, 141.
,blind in cave, 137.
Rattle-snake, 201.
Reason and instinct, 208.
Recapitulation, general, 459.
Reciprocity of crosses, 258.
Record, geological, imperfect, 279.
Rengger on flies destroying cattle, 72,
Reproduction, rate of, 63.
Resemblance to parents in mongrels andhybrids, 273.
Reversion, law of inheiitance, 14.
-in pigeons to blue colour, 160.
Rhododendron, sterility of, 251.
Richard, Prof., on Aspiearpa, 417.
Richardson, Sir J., on structure ofsquirrels, 180.
,on fishes of the southern hemisphere, 376.
Robiuia, grafb of, 262.

--

INDEX.
~RODENTS.

STRIPES.

Rodents, blind, 137.
Rudimentary organs, 450.
Rudiments important for classification,
416.

Silene, fertility of crosses, 257.
Silliman, Prot., on blind rat, 137.
Skulls of young mammals, 197, 437.
Slave-making instinct, 219.
Smith, Col. Hamilton, on striped horses,
164.
, Mr. Fred., on slave-making ants,
219.
-, on neuter ants, 239.
-,
Mr., of Jordan Hill, on the degradation of coast-rocks, 283.
Snapdragon, 16 1.
Somerville, Lord, on selection of sheep,
31.
Sorbus, grafts of, 262.
Spaniel, King Charles’s breed, 35.
Species, polymorphic, 46.
-, common, variable, 53.
in large genera variable, 54.
-,
groups of, suddenly appearing,
302, 306.
-beneath Silurian formations, 306.
successively appearing, 312.
changing simultaneously throughout the world, 323.
Spencer, Lord, on increase in size ofcattle, 35.
Sphex, parasitic, 218.
Spiders, development of, 442.
Spitzdog crossed with fox, 268.
Sports in plants, 9.
Sprengel, C. C., on crossing, 98.
-, on ray-florets, 145.
Squirrels, gradations in structure, 180.
Staffordshire, heath, changes in, 72.
Stag-beetles, fighting, 88.
Sterility from changed conditions oflife, 9.
of hybrids, 246.
-, laws of, 254.
-, causes of, 263.
from unfavonrable conditions,265.
of certain varieties, 269.
St. Helena, productions of, 389.
St. Hilaire, Aug., on classification, 418.
St. John, Mr., on habits of cats, 91.
Sting of bee, 202.
Stocks, aboriginal, of domestic animals,
18.
Strata, thickness of, in Britain, 284.
Stripes on horses, 163.

8.

Sageret on grafts, 262.
Salmons, males fighting, and hooked
jaws of, 88.
Salewater, how far injurious to seeds,
358.
Saurophagus sulphuratus, 183.
Schiodte on blind insects, 138.
Schlegel on snakes, 144.
Sea-water, how far injurious to seeds,
358.
Sebright, Sir J., on crossed animals,
20.
, on selection of pigeons, 31.
Sedgwick, Prof., on groups of species
suddenly appearing, 302.
Seedlings destroyed by insects, 67.
Seeds, nutriment in, 77.
, winged, 146.
, power of resisting salt - water,358.
in crops and intestines of birds,
361.
eaten by fish, 362, 387.
in mud, 386.
-, hooked, on islands, 392.
Selection of domestic products, 29.
-,
principle not of recent origin,33.
, unconscious, 34.
, natural, 80.
-, sexual, 87.
-, natural, circumstances favourable
to, 101.
Sexes, relations of, 87.
Sexual characters variable, 156.
selection, 87.
Sheep, Merino, their selection, 31.
, two sub-breeds unintentionallyproduced, 36.
-, mountain, varieties of, 76.
Shells, colours of, 132.
-,
littoral, seldom embedded, 288.
-, fresh-water, dispersal of, 385.
of Madeira, 391.
-, land, distribution of, 397.

-

-

-

-

-

-

-

-

-

-

501

INDEX.

STRUCTURE.

VIOLA.

Structure, degrees of utility of, 201.
Struggle for existence, 60.
Succession, geological, 312.
Succession of types in same arms, 338.
Swallow, one species supplanting another, 76.
Swim-blahher, 190.
System, natural, 413.

Tumbler pigeons, habits of, hereditary,214.
-, pU"g of, 446.
Turkey-cock, brush of hair on breast,
90.
Turkey, naked skin on head, 197.
,young, wila, 216.
Turnip and cabbage, analogous variationsof, 159.
Type, unity of, 206.
Types, succession of, in same areas, 338.

T.
Tail of giraffe, 195.
of aquatic animals, 196.
-,
rudimentary, 454.
Tarsi deficient, 135.
Tausch on umbelliferous flowers, 146.
Teeth and hair correlated, 144.
, embryonic, traces of, in birds,
451.
-, rudimentary, in embryonic calf,
450, 480.
Tegetmeier, Mr., on cells of bees, 228,
233.
Temminck on distribution aiding classification, 419.
Thouin on grafts, 262.
Thrush, aquatic species of, 185.
-, mocking, of the Galapagos, 402.
-, young of, spotted, 439.
-, nest of, 243.
Thuret, M., on crossed fuci, 258.
Thwaites, Mr., on acclimatisation, 140.
Tierra del Puego, dogs of, 215.
-, plants of, 374, 378.
Timber-drift, 360.
Time, lapse of, 282.
Titmouse, 183.
Toads on islands, 593.
Tobacco, crossed varieties of, 271.
Tomes, Mr., on the distribution of hats,
394.
Transitions in varieties rare, 172.
Trees on islands belong to peculiar
orders, 392.
-withseparatd sexes, 99.
Trifolium pratense, 73, 94.
incarnatum, 94.
Trigonia, 321.
Trilobites, 306.
-, sudden extinction of, 321.
Troglodytes, 243.
Tucutucu, blind, 137.

-

-

-

-

0.
Udders enlarged by use, 11.
,rudimentary, 451.
Ulex, young leaves of, 439.
Umbellifere, outer and inner florets of,144.
Unity of type, 206.
Use, effects of, under domestication, 11.
-, effects of, in a state of nature,
134.
Utility, bow far important in the construction of each part, 199.

-

V.
Valenciennes on fresh-water fish, 384.
Variability of mongrels and hybrids,
274.
Variation under domestication, 7.
caused by reproductive systembeing affected by conditions of life, 8.
under nature, 44.
-_ ,laws of, 131.
Variations appear at corresponding ages,
14, 86.
-,analogous in distinct species,
159.
Varieties, natural, 44.
,struggle between, 75.
-,domestic, extinction of, 111.
-, transitional, rarity of, 172.
-, when crossed, fertile, 267.
-, when crossed, sterile, 269.
-, classification of, 423.
Verbascum, sterility of, 251.
-, varirties of, crossed, 270.
Verneuil, M. de, on the succession of
species, 325.
Viola tricolor, 73.

-

_
.

502

INDEX.

VOLCANIC.

ZEBRA.

Volcanic islands, denudation of, 284.
Vulture, naked skin on head, 197.

Wings of insects homologous with branchi=, 191.
-, rudimentary, in insects, 451.
Wolf crossed with dog, 214.
-of Falkland Isles, 393.
Wollaston, Mr., on varieties of insects,

W.
Wading-birds, 386.
Wallace, Mr., on origin of species, 2.
-, on law of geographical distribution, 355.
-, on the Malay Archipelago, 395.
Wasp, sting of, 202.
Water, fresh, productions of, 383.
Water-hen, 185.
Waterhouse, Mr., on Australian marsupials, 116.
-, on greatly developed parts being
variable, 150.
-, on the cells of bees, 225.
-, on general affinities, 429.
Water-ouzel, 185.
Watson, Mr. H.C., on range of varietiesof British plants, 58.
-, on acclimatisation, 140.
-, on flora of Azores, 363.
-, on Alpine plants, 367, 376.
-, on rarity of intermediate varieties, 176.
Weald, denudation of, 285.
Web of feet in water-birds, 185.
West Indian islands, mammals of, 395.
W e s t w d on species in large genera
being closely allied to others, 57.
on the tarsi of Engidz, 157.
on the antennae of hymenopterous
insects, 416.
Whales, fossil, 303.
Wheat, varieties of, 113.
White Mountains, flora of, 365.
Wings, reduction of size, 134.

-48.

-, on fossil varieties of land-shells
in Madeira, 52.
-,on colours of insects on sea-shore,132.
-,on wingless beetles, 185.
-, on rarity of intermediate varieties, 176.
on insular insects, 389.
on land-shells of Madeira, natuialised, 402.
Wolves, varieties of, 90.
Woodpecker, habits of, 184.
-, green colour of, 197.
Woodward, Mr., on the duration of
specific forms, 293.
-, on the continuous succession ofgenera, 316.
-, on the succession of types, 339.
World, species changing simultaneously
thoughout, 322.
Wrens, nest of, 243.

-,
-,

Y.
Youatt, Mr., on selection, 31.
,on sub-breeds of sheep, 36.
-,on rudimentary horns in young
cattle, 454.

-

2.
Zebra, stripes on, 163.

THE END.

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